ci: bump golangci-lint to v2.12.2 to fix Go 1.26 toolchain panic (#761)

The pinned golangci-lint v2.1.6 (built with Go 1.24) panics when
analyzing code with the Go 1.26 toolchain installed by setup-go,
since go/types refuses to type-check files requiring a newer Go
version than the one golangci-lint itself was built with:

  panic: file requires newer Go version go1.26 (application built with go1.24)

Bumped all 4 workflow files that pin golangci-lint (test.yml,
test_gc_opt.yml, test_poll_opt.yml, test_poll_opt_gc_opt.yml) to
v2.12.2, which is built with Go 1.26.2.

Verified locally: golangci-lint v2.12.2 run with the same linters/args
as CI (-E gocritic -E misspell -E revive -E godot, with/without
--timeout 5m) reports 0 issues on the repo.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: panjf2000

.github/workflows/test.yml

@@ -49,7 +49,7 @@ jobs:
       - name: Setup and run golangci-lint
         uses: golangci/golangci-lint-action@v7
         with:
-          version: v2.1.6
+          version: v2.12.2
           args: -v -E gocritic -E misspell -E revive -E godot --timeout 5m
   test:
     needs: lint

.github/workflows/test_gc_opt.yml

@@ -49,7 +49,7 @@ jobs:
       - name: Setup and run golangci-lint
         uses: golangci/golangci-lint-action@v7
         with:
-          version: v2.1.6
+          version: v2.12.2
           args: -v -E gocritic -E misspell -E revive -E godot --timeout 5m
   test:
     needs: lint

.github/workflows/test_poll_opt.yml

@@ -48,7 +48,7 @@ jobs:
       - name: Setup and run golangci-lint
         uses: golangci/golangci-lint-action@v7
         with:
-          version: v2.1.6
+          version: v2.12.2
           args: -v -E gocritic -E misspell -E revive -E godot
   test:
     needs: lint

.github/workflows/test_poll_opt_gc_opt.yml

@@ -48,7 +48,7 @@ jobs:
       - name: Setup and run golangci-lint
         uses: golangci/golangci-lint-action@v7
         with:
-          version: v2.1.6
+          version: v2.12.2
           args: -v -E gocritic -E misspell -E revive -E godot
   test:
     needs: lint