xbps-src: use $XBPS_GIT_CMD for git

xbps-src

@@ -508,14 +508,13 @@ fi
 
 # Try using chroot-git then git from the host system
 if command -v chroot-git &>/dev/null; then
-    XBPS_GIT_CMD=$(command -v chroot-git)
+    export XBPS_GIT_CMD=$(command -v chroot-git)
 elif command -v git &>/dev/null; then
-    XBPS_GIT_CMD=$(command -v git)
+    export XBPS_GIT_CMD=$(command -v git)
 else
     echo "neither chroot-git or git are available in your system!" 1>&2
     exit 1
 fi
-readonly XBPS_GIT_CMD
 
 if [ -n "$XBPS_HOSTDIR" ]; then
     export XBPS_REPOSITORY=$XBPS_HOSTDIR/binpkgs