gha: Use 'oldstable' as the go version

Getting the go version from the go.mod file is nice in theory, but has a
number of caveats:
- we are not always pro-active in updating it, so we could be using an
  old unsupported version in CI
- if the version in go.mod is older than the version in tools/go.mod,
  there can be issues when trying to build golangci-lint
- this does not interact nicely with recent changes in go.mod and the
  introduction of the toolchain directive, as we are now specifying the
  minor go version (1.22.0) in go.mod, but we want to build with the
  latest released 1.22 go version.

For all these reasons, this commit switches back to using `oldstable` as
the go version to build with instead of getting it from go.mod

`oldstable` currently points at 1.23, see
https://github.com/actions/setup-go?tab=readme-ov-file#using-stableoldstable-aliases
for a longer explanation.

Signed-off-by: Christophe Fergeau <[email protected]>

.github/workflows/macadam.yml

@@ -52,7 +52,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v5
         with:
-          go-version-file: 'go.mod'
+          go-version: 'oldstable'
       - name: Test
         run: make test
   lint:
@@ -66,6 +66,6 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v5
         with:
-          go-version-file: 'go.mod'
+          go-version: 'oldstable'
       - name: Run 'make check'
         run: make check