tool/gocross: correctly embed the git commit into gocross

Previously, the build ended up embedding an empty string, which made
the shell wrapper rebuild gocross on every invocation. This is still
reasonably fast, but fixing the bypass shaves 80% off gocross's overhead
when no rebuild is needed.

Signed-off-by: David Anderson <danderson@tailscale.com>

tool/gocross/gocross-wrapper.sh

@@ -82,9 +82,9 @@ fi
 # Anyway, build gocross in a stripped down universe.
 gocross_path="$repo_root/gocross"
 gocross_ok=0
+wantver="$(git rev-parse HEAD)"
 if [ -x "$gocross_path" ]; then
 	gotver="$($gocross_path gocross-version 2>/dev/null || echo '')"
-	wantver="$(git rev-parse HEAD)"
 	if [ "$gotver" = "$wantver" ]; then
 		gocross_ok=1
 	fi
@@ -95,7 +95,7 @@ if [ "$gocross_ok" = "0" ]; then
     unset GO111MODULE
     unset GOROOT
     export CGO_ENABLED=0
-    "$toolchain/bin/go" build -o "$gocross_path" -ldflags='-X tailscale.com/version/gitCommitStamp=$wantver' tailscale.com/tool/gocross
+    "$toolchain/bin/go" build -o "$gocross_path" -ldflags "-X tailscale.com/version.gitCommitStamp=$wantver" tailscale.com/tool/gocross
 fi
 ) # End of the subshell execution.