ci: pin Go 1.26 in test workflows via setup-go (#2920)

* Initial plan

* ci: add explicit setup-go step to use Go 1.26 matching go.mod

Both testing.yml and testing-git.yml were relying on whatever Go version
shipped in ghcr.io/jguer/yay-builder:latest (Go 1.24.13), which is
incompatible with the go 1.26 / toolchain go1.26.0 declarations in
go.mod and scripts/go.mod, causing build/lint failures in PR #2917.

Add an actions/setup-go step (pinned to the same hash already used in
pages.yml) immediately after checkout so CI always picks up Go 1.26 as
read from go.mod, regardless of what the builder image contains.

Co-authored-by: Jguer <8071073+Jguer@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Jguer <8071073+Jguer@users.noreply.github.com>

.github/workflows/testing-git.yml

@@ -15,6 +15,9 @@ jobs:
       image: ghcr.io/jguer/yay-builder:latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5.6.0
+        with:
+          go-version-file: go.mod
       - uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
         with:
           path: ~/go/pkg/mod