[tailscale] runtime/debug: embed Tailscale toolchain git rev

This is another take on #49 (which we stopped using?), and less
intrusive, and also always on.

This puts puts the the Tailscale Go toolchain's git rev in
the binary, accessible as runtime/debug.TailscaleGitRev.

Caller code will have to access it guarded by the "tailscale_go" build
tag.

Updates #49

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/build.yml

@@ -41,6 +41,8 @@ jobs:
       uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
       with:
         ref: ${{ inputs.ref || github.ref }}
+    - name: set runtime/debug.TailscaleGitRev
+      run: sed -i "s/TAILSCALE_GIT_REV_TO_BE_REPLACED_AT_BUILD_TIME/${{ github.sha }}/" src/runtime/debug/tailscale-git-rev.txt
     - name: build
       run: cd src && ./make.bash
       env:

src/runtime/debug/mod.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 )
 
+const TailscaleGitRev = `TAILSCALE_GIT_REV_TO_BE_REPLACED_AT_BUILD_TIME`
+
 // exported from runtime.
 func modinfo() string