feat: add go-download-base-url input for custom Go distributions (#721)

* feat: add go-download-base-url input for custom Go distributions

Add support for downloading Go from custom sources such as Microsoft Go
(aka.ms). Users can specify a custom download base URL via the
`go-download-base-url` input or the `GO_DOWNLOAD_BASE_URL` environment
variable (input takes precedence).

When a custom URL is provided, the action skips the GitHub-hosted
manifest and attempts to resolve versions from the custom URL's JSON
listing. If the listing is unavailable (as with aka.ms redirect links),
it falls back to constructing the download URL directly from the
version, platform, and architecture.

Usage:
  - uses: actions/setup-go@v6
    with:
      go-version: '1.25'
      go-download-base-url: 'https://aka.ms/golang/release/latest'

Changes:
- action.yml: add go-download-base-url optional input
- installer.ts: add getInfoFromDirectDownload() for URL construction
  fallback, thread custom URL through getGo/getInfoFromDist/findMatch
- main.ts: read new input and GO_DOWNLOAD_BASE_URL env var
- setup-go.test.ts: add 12 unit tests for custom URL behavior
- microsoft-validation.yml: add E2E workflow testing Microsoft build of Go
  across ubuntu/windows/macos with versions 1.24 and 1.25
- README.md: document new input with Microsoft build of Go examples

* run prettier

* fixup PR review

* revert cache-save

* fixup

* handle distinct cache

* skip json for known URL

* fix bug in JSON with custom URL

docs/advanced-usage.md

@@ -12,6 +12,7 @@
   - [Restore-only caches](advanced-usage.md#restore-only-caches)
   - [Parallel builds](advanced-usage.md#parallel-builds)
 - [Outputs](advanced-usage.md#outputs)
+- [Custom download URL](advanced-usage.md#custom-download-url)
 - [Using `setup-go` on GHES](advanced-usage.md#using-setup-go-on-ghes)
 
 ## Using the `go-version` input
@@ -222,6 +223,8 @@ want the most up-to-date Go version to always be used. It supports major (e.g.,
 
 > Setting `check-latest` to `true` has performance implications as downloading Go versions is slower than using cached
 > versions.
+>
+> `check-latest` is ignored when `go-download-base-url` is set. See [Custom download URL](#custom-download-url) for details.
 
 ```yaml
 steps:
@@ -417,6 +420,57 @@ jobs:
       - run: echo "Was the Go cache restored? ${{ steps.go124.outputs.cache-hit }}" # true if cache-hit occurred
 ```
 
+## Custom download URL
+
+The `go-download-base-url` input lets you download Go from a mirror or alternative source instead of the default `https://go.dev/dl`. This can also be set via the `GO_DOWNLOAD_BASE_URL` environment variable; the input takes precedence over the environment variable.
+
+When a custom base URL is provided, the action skips the `actions/go-versions` manifest lookup and downloads directly from the specified URL.
+
+**Using the [Microsoft build of Go](https://github.com/nicholasgasior/microsoft-go):**
+
+```yaml
+steps:
+  - uses: actions/checkout@v6
+  - uses: actions/setup-go@v6
+    with:
+      go-version: '1.25'
+      go-download-base-url: 'https://aka.ms/golang/release/latest'
+  - run: go version
+```
+
+**Using an environment variable:**
+
+```yaml
+env:
+  GO_DOWNLOAD_BASE_URL: 'https://aka.ms/golang/release/latest'
+
+steps:
+  - uses: actions/checkout@v6
+  - uses: actions/setup-go@v6
+    with:
+      go-version: '1.25'
+  - run: go version
+```
+
+> **Note:** Version range syntax (`^1.25`, `~1.24`, `>=1.25.0`) and aliases (`stable`, `oldstable`) are not supported with custom download URLs. Use exact versions such as `1.25`, `1.25.0`, or `1.25.0-1` (for sources that use revision numbers). If the custom server provides a version listing endpoint (`/?mode=json&include=all`), semver ranges will work; otherwise only exact versions are accepted.
+
+> **Note:** The `check-latest` option is ignored when a custom download base URL is set. The action cannot query the custom server for the latest version, so it uses the version you specify directly. If you provide a partial version like `1.25`, the server determines which patch release to serve.
+
+**Authenticated downloads:**
+
+If your custom download source requires authentication, the `token` input is forwarded as an `Authorization` header. For example, to download from a private mirror:
+
+```yaml
+steps:
+  - uses: actions/checkout@v6
+  - uses: actions/setup-go@v6
+    with:
+      go-version: '1.25'
+      go-download-base-url: 'https://private-mirror.example.com/golang'
+      token: ${{ secrets.MIRROR_TOKEN }}
+  - run: go version
+```
+
 ## Using `setup-go` on GHES
 
 ### Avoiding rate limit issues