.github/workflows: fix timeouts by caching packages for golangci-lint (#18398)

Recently, the golangci-lint workflow has been taking longer and longer
to complete, causing it to timeout after the default of 5 minutes.

    Running error: context loading failed: failed to load packages: failed to load packages: failed to load with go/packages: context deadline exceeded
    Timeout exceeded: try increasing it by passing --timeout option

This PR upgrades actions/setup-go to version 6, the latest, and
enables caching for Go modules and build outputs. This should speed up
linting because most packages won’t have to be downloaded over and
over again.

Fixes #18366

Signed-off-by: Simon Law <sfllaw@tailscale.com>

.github/workflows/golangci-lint.yml

@@ -29,10 +29,10 @@ jobs:
     steps:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
 
-      - uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5.5.0
+      - uses: actions/setup-go@4dc6199c7b1a012772edbd06daecab0f50c9053c # v6.1.0
         with:
           go-version-file: go.mod
-          cache: false
+          cache: true
 
       - name: golangci-lint
         uses: golangci/golangci-lint-action@1481404843c368bc19ca9406f87d6e0fc97bdcfd # v7.0.0