build: generate the fd.o source tarballs locally instead of fetching archives

gitlab.freedesktop.org's on-demand archive endpoint 503/504s for hours
at a stretch: runs #25, #27, #30 and #31 all died in 'abuild checksum'
fetching the pinned libqmi tarball, outlasting the 3x30s in-run retry.
git clone is served from a different code path and holds up (verified
against all three repos while the archive endpoint was flaky), so clone
libqmi/ModemManager/libcamera (retried) and git-archive the pinned refs
into the aports, the same way the kernel and imsd tarballs are already
produced. The pins stay single-sourced in the APKBUILDs.

libcamera gains a checksum step: its committed sums were for the
fd.o-served tarball, and git-archive output is not byte-identical.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

aports/temp/modemmanager/APKBUILD

@@ -54,7 +54,9 @@ subpackages="
 	$pkgname-udev
 	"
 #source="https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/archive/$pkgver/ModemManager-$pkgver.tar.gz
-source="https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/archive/$_commit/ModemManager-$_commit.tar.gz
+# fp6-img: local tarball, generated by build.sh from a git clone — fd.o's
+# archive endpoint 504s for hours at a stretch (killed CI runs 27/30/31)
+source="ModemManager-$_commit.tar.gz
 	$pkgname.rules
 	$pkgname.initd