Force PGP signature file extension

archiso/mkarchiso:
Force the file extension in use for the PGP signatures of the rootfs to always be .sig.
When gnupg's 'armor' configuration option is used, the output otherwise defaults to using .asc.
As the verification hook in mkinitcpio-archiso expects the .sig file extension, verifying the rootfs will fail in that
scenario.

archiso/mkarchiso

@@ -227,10 +227,11 @@ _mkchecksum() {
 _mksignature() {
     _msg_info "Signing SquashFS image..."
     cd -- "${isofs_dir}/${install_dir}/${arch}"
+    # always use the .sig file extension, as that is what mkinitcpio-archiso's hooks expect
     if [[ -e "${isofs_dir}/${install_dir}/${arch}/airootfs.sfs" ]]; then
-        gpg --detach-sign --default-key "${gpg_key}" airootfs.sfs
+        gpg --output airootfs.sfs.sig --detach-sign --default-key "${gpg_key}" airootfs.sfs
     elif [[ -e "${isofs_dir}/${install_dir}/${arch}/airootfs.erofs" ]]; then
-        gpg --detach-sign --default-key "${gpg_key}" airootfs.erofs
+        gpg --output airootfs.erofs.sig --detach-sign --default-key "${gpg_key}" airootfs.erofs
     fi
     cd -- "${OLDPWD}"
     _msg_info "Done!"