CI: build: fix external toolchain use with release tag tests

CI: build: fix external toolchain use with release tag tests

When a new tag for a release is created, the just checkout repo from
github actions will already have such tag locally created.

This will result in git fetch --tags failing with error rejecting the
remote tag with (would clobber existing tag).

Add -f option to overwrite any local tags and always fetch them from
remote.

Fixes: e24a1e6f6d7f ("CI: build: add support for external toolchains from stable branch")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>