.github/actions/go-cache: check for pre-built cigocacher (#18833)

Some CI runner images now have cigocacher baked in. Skip building if
it's already present.

Updates tailscale/corp#35667

Change-Id: I5ea0d606d44b1373bc1c8f7bca4ab780e763e2a9

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

.github/actions/go-cache/action.sh

@@ -23,6 +23,10 @@ if [ -z "${URL:-}" ]; then
     exit 0
 fi
 
+BIN_PATH="$(PATH="$PATH:$HOME/bin" command -v cigocacher || true)"
+if [ -z "${BIN_PATH}" ]; then
+  echo "cigocacher not found in PATH, attempting to build or fetch it"
+
   GOPATH=$(command -v go || true)
   if [ -z "${GOPATH}" ]; then
     if [ ! -f "tool/go" ]; then
@@ -40,6 +44,7 @@ else
     echo "cmd/cigocacher not found locally, fetching from tailscale.com/cmd/cigocacher"
     "${GOPATH}" build -o "${BIN_PATH}" tailscale.com/cmd/cigocacher
   fi
+fi
 
 CIGOCACHER_TOKEN="$("${BIN_PATH}" --auth --cigocached-url "${URL}" --cigocached-host "${HOST}" )"
 if [ -z "${CIGOCACHER_TOKEN:-}" ]; then