workflows: Fix build-dist tarball name for PRs
workflows: Fix build-dist tarball name for PRs

actions/checkout uses the `pull/NNN/head` ref by default instead of the
proposed branch's HEAD. This lead to wrong tarball names:
The SHA in `cockpit-*.g<SHA>.tar.xz` did not match the artifact's
`dist-<SHA>` name and moreover not the local developer's `git describe`,
so that it is impossible to check whether the dist tarball for the
current local version was already downloaded.

Tell the checkout action to explicitly check out the PR's proposed
branch, exactly like in naming the artifact.

martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit