.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

@@ -23,22 +23,27 @@ if [ -z "${URL:-}" ]; then
     exit 0
 fi
 
-GOPATH=$(command -v go || true)
-if [ -z "${GOPATH}" ]; then
+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
       echo "Go not available, unable to proceed"
       exit 1
     fi
     GOPATH="./tool/go"
-fi
+  fi
 
-BIN_PATH="${RUNNER_TEMP:-/tmp}/cigocacher$(${GOPATH} env GOEXE)"
-if [ -d "cmd/cigocacher" ]; then
+  BIN_PATH="${RUNNER_TEMP:-/tmp}/cigocacher$(${GOPATH} env GOEXE)"
+  if [ -d "cmd/cigocacher" ]; then
     echo "cmd/cigocacher found locally, building from local source"
     "${GOPATH}" build -o "${BIN_PATH}" ./cmd/cigocacher
-else
+  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}" )"