linux-postmarketos-allwinner: fetch tarball from GitHub mirror

The codeberg tarball fetch was timing out the verify-checksums CI job
(no bytes received after TCP/TLS handshake for 15 minutes). Switch to
Arnav Singh's GitHub mirror of megi's tree, which mirrors the same tag
(orange-pi-6.18-20260105-0049).

The recorded sha512sum changes because GitHub and Gitea (codeberg)
generate tarballs with different bytes for the same git tag.

Also adjust builddir, since GitHub's tarball top-level directory is
named "<repo>-<tag>" (linux-orange-pi-6.18-20260105-0049/) where
codeberg's was just "<repo>" (linux/).

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8501>

device/community/linux-postmarketos-allwinner/APKBUILD

@@ -32,7 +32,7 @@ options="!strip !check !tracedeps
 	pmb:cross-native
 	pmb:kconfigcheck-community
 	"
-source="$pkgname-$_tag.tar.gz::https://codeberg.org/megi/linux/archive/$_tag.tar.gz
+source="$pkgname-$_tag.tar.gz::https://github.com/Arnavion/linux/archive/refs/tags/$_tag.tar.gz
 	config-$_flavor.aarch64
 	config-$_flavor.armv7
 	0001-dts-add-dontbeevil-pinephone-devkit.patch
@@ -48,7 +48,7 @@ source="$pkgname-$_tag.tar.gz::https://codeberg.org/megi/linux/archive/$_tag.tar
 	0011-arm64-dts-allwinner-orangepi-3-fix-ethernet.patch
 	0012-staging-rtl8723cs-fix-CFI-signature-of-rtw_xmit_ent.patch
 	"
-builddir="$srcdir/linux"
+builddir="$srcdir/linux-$_tag"
 
 case "$CARCH" in
 	aarch64*) _carch="arm64" ;;
@@ -84,7 +84,7 @@ package() {
 		INSTALL_DTBS_PATH="$pkgdir/boot/dtbs"
 }
 sha512sums="
-377282b4bfa1a0ec8e0a68df3e3d7d92c71cebe80730770e13880b4cf384f95e92db3e79a43c71e95315d1647ef970b0bfe139a5f607e23b623d771f5ce65347  linux-postmarketos-allwinner-orange-pi-6.18-20260105-0049.tar.gz
+3f291658f6e9183d4a7c56c96bd6b6c84f16c3ce96c023a78b80695d90e029de53a19126b34cd770d72fc67204d59bb0f16c9a1a87876cdaa4d5d21f18dc6c49  linux-postmarketos-allwinner-orange-pi-6.18-20260105-0049.tar.gz
 6b3254e0d44570d603c9a66941fcb47750e1999691218d749b4b6f29af44fa3de73ba72b5115b9136c852ec4c17cd3d3c91a6422bb54d729b9189673d3001c44  config-postmarketos-allwinner.aarch64
 f12b53ed42811fc7fa90d7efda9372bb2e10c649f433b4eca2a9c2c7e9dc0b3e82450c3c94e18fcf287199d8aa4a5358daf005fbbaf363bdea9ad215a12241ec  config-postmarketos-allwinner.armv7
 5ee446b124ae551fab659fe51e4bf9a617218f464398ed899bb438c469e93bacc248b1a6d3936ac36ba812971170696765117f2b619fdd37e5ff21b46f01bde9  0001-dts-add-dontbeevil-pinephone-devkit.patch