Read PKGBUILD variables with CARCH set and refuse to queue a package whose version cannot be read

makepkg always exports CARCH, so PKGBUILDs may branch on it at file
scope. Every place the tooling sourced a PKGBUILD did so without CARCH,
taking the wrong branch or aborting partway, and check-versions turned
the resulting empty pkgver/pkgrel into the version '-', which never
matches a published version and queues an endless rebuild.

package_pkgbuild_var reads one variable the way makepkg would see it,
for the architecture being checked, and reports whether the source
succeeded. check-versions, sync-rebuilds and omarchy-pkgs use it.
check-versions now warns and skips a package whose pkgver or pkgrel is
empty instead of comparing a partial version. A self-test covers a
PKGBUILD that branches on CARCH before assigning its version.

bin/omarchy-pkgs

@@ -21,6 +21,7 @@ BUILD_ROOT=$(realpath "${BASH_SOURCE[0]%/*}/..")
 source "$BUILD_ROOT/helpers/message-helpers.sh"
 source "$BUILD_ROOT/helpers/paths.sh"
 source "$BUILD_ROOT/helpers/host-helpers.sh"
+source "$BUILD_ROOT/helpers/package-metadata.sh"
 
 UPSTREAM_URL="${OMARCHY_UPSTREAM_URL:-https://github.com/basecamp/omarchy.git}"
 EDGE_DB_URL="${OMARCHY_EDGE_DB_URL:-https://pkgs.omarchy.org/edge/$(reference_arch)/omarchy.db.tar.zst}"
@@ -92,7 +93,7 @@ version_is_rc() { version_is_prerelease "$1"; }
 
 pkgbuild_var() {
   local pkg="$1" var="$2"
-  (cd "$BUILD_ROOT/pkgbuilds/$pkg" && bash -c "source PKGBUILD 2>/dev/null; echo \"\${$var}\"")
+  package_pkgbuild_var "$BUILD_ROOT/pkgbuilds/$pkg" "$var"
 }
 
 # Prints the published version of $pkg from the edge DB. Distinguishes