catalyst-auto: automatically calculate REPO_DIR

Rather than hardcode it to a path that works on only a few systems,
switch it to the dynamic calculation like we use for GITDIR.

config/build.env

@@ -1,5 +1,4 @@
 BASE_DIR="/release"
-REPO_DIR="/release/releng"
 TMP_PATH="/release/tmp/run"
 BUILD_SRCDIR_BASE=${BASE_DIR}/buildroot/${ARCH}-dev/builds/
 BUILD_DESTDIR_BASE=${BASE_DIR}/weekly/builds/${ARCH}/

tools/catalyst-auto

@@ -159,8 +159,9 @@ if [[ -n ${lock_file} ]]; then
   fi
 fi
 
-# Probe the default gitdir from this script name.
-GITDIR=$(dirname "$(dirname "$(realpath "$0")")")
+# Probe the default source dir from this script name.
+REPO_DIR=$(dirname "$(dirname "$(realpath "$0")")")
+GITDIR=${REPO_DIR}
 
 # Set up defaults that config files can override if they want.
 SUBARCH=$(uname -m)