fix(ci): import public key for checksig and force dist tag override

Import the GPG public key into rpm's keyring so rpm --checksig can
verify signatures. Also use --undefine dist before --define to ensure
the CLI value overrides the system macro on the build host.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-release.yml

@@ -100,6 +100,7 @@ jobs:
           rpmbuild -bb rpm/mistralrs.spec \
             --define "mistralrs_version ${version}" \
             --define "mistralrs_flavour ${{ matrix.name }}" \
+            --undefine dist \
             --define "dist .fc${{ matrix.fedora_version }}"
         env:
           TAG: ${{ inputs.tag }}