.github/workflows: force use of local Go toolchain

As of Go version 1.21, the "go" command may, under certain circumstances,
fetch a different version of the Go toolchain than is provided locally, as
described in git-lfs/git-lfs#5477, PR git-lfs/build-dockers#58, and the
Go documentation:

https://go.dev/doc/toolchain

In general, we do not expect this to occur, as we specify an older
Go version in our go.mod file and so the locally-installed Go toolchain
should be newer than that version and therefore sufficient.

However, to be cautious and ensure we only try to run with the Go version
we have specified, we add the GOTOOLCHAIN=local environment variable
to all our GitHub Actions workflow jobs.  Defining this variable
will guarantee that Go will stop with an error rather than proceed to
download and run another version of itself.

Author: chrisd8088

.github/workflows/ci.yml

@@ -1,5 +1,7 @@
 name: CI
 on: [push, pull_request]
+env:
+  GOTOOLCHAIN: local
 
 jobs:
   build-go: