.github/workflows: build golangci-lint from source [DO NOT MERGE]

The prebuilt golangci-lint v2.10.1 binary is built with Go 1.26 and
refuses to target Go 1.27rc2 ('the Go language version used to build
golangci-lint is lower than the targeted Go version'). Use the
action's goinstall mode so it's built with the Go 1.27 RC toolchain
installed by setup-go. None of our enabled linters bundle staticcheck,
so building from source should be enough.

Updates #20220

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ie4ef54d1254576c4b4b6643f501103ce3117e2b4

.github/workflows/golangci-lint.yml

@@ -41,6 +41,11 @@ jobs:
         with:
           version: v2.10.1
 
+          # DO NOT MERGE: the prebuilt golangci-lint binary is built with
+          # Go 1.26 and refuses to target Go 1.27rc2, so build it from
+          # source with the Go toolchain installed above.
+          install-mode: goinstall
+
           # Show only new issues if it's a pull request.
           only-new-issues: true