fix(ci): POSIX shell in deb/rpm Version step (dash "Bad substitution")

deb.yml runs in the Ubuntu rust-ci image whose /bin/sh is dash, where the bash
substring `${GITHUB_SHA::8}` is a "Bad substitution" — the deb build failed at the
Version step every run. Compute the short SHA with `cut` instead. (rpm.yml ran fine
because the Fedora image's /bin/sh is bash, but fix it the same way for robustness.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/rpm.yml

@@ -46,9 +46,10 @@ jobs:
         # sorts BEFORE the eventual "1" yet increases by run number, so `rpm-ostree upgrade`
         # always moves to the newest main build.
         run: |
+          SHORT=$(echo "$GITHUB_SHA" | cut -c1-8)
           case "$GITHUB_REF" in
             refs/tags/v*) V="${GITHUB_REF_NAME#v}"; R="1" ;;
-            *)            V="0.0.1"; R="0.ci${GITHUB_RUN_NUMBER}.g${GITHUB_SHA::8}" ;;
+            *)            V="0.0.1"; R="0.ci${GITHUB_RUN_NUMBER}.g${SHORT}" ;;
           esac
           echo "PF_VERSION=$V" >> "$GITHUB_ENV"
           echo "PF_RELEASE=$R" >> "$GITHUB_ENV"