Make CI jobs use the local Go toolchain

Previously the test:fips and other CI jobs downloaded and used the standard Go toolchain listed in go.mod. But we really want to build and test against the Go compiler provided in the build image, so set GOTOOLCHAIN=local accordingly.

This came out of https://gitlab.com/gitlab-org/build/team-tasks/-/issues/13#note_2699786297.

For example, previously in the Go 1.23 CI job in https://gitlab.com/gitlab-org/gitlab-shell/-/jobs/11070468776:

$ go version
go version go1.24.5 linux/amd64

Now in https://gitlab.com/gitlab-org/gitlab-shell/-/jobs/11088673693:

$ go version
go version go1.23.9 linux/amd64