[tailscale1.20] cmd/dist: support embedding of toolchain rev by env var

Git checkouts are not reproducible (in particular, the .git directory's hashes
vary according to the moods of git re: packing and other local settings), so
building our Go toolchain via Nix leads to source hashes that change from machine
to machine and over time.

Since Nix already knows the exact git revision it's building from, this change
lets Nix provide that hash to cmd/dist directly, skipping the rooting around in
.git to find the right hash.

Signed-off-by: David Anderson <danderson@tailscale.com>
(cherry picked from commit 4cbd268)

src/cmd/dist/build.go

@@ -364,6 +364,11 @@ func findgoversion() string {
 			}
 			if isGitRepo() {
 				b += "-ts" + chomp(run(goroot, CheckExit, "git", "log", "-n", "1", "--format=format:%h", "HEAD"))
+			} else if rev := os.Getenv("TAILSCALE_TOOLCHAIN_REV"); rev != "" {
+				if len(rev) > 10 {
+					rev = rev[:10]
+				}
+				b += "-ts" + chomp(rev)
 			}
 			return b
 		}