contrib: fix build_locale.sh to work with relative paths

I think all scripts that call this file already used abs paths,
but manual callers tend to use relative paths.

contrib/build_locale.sh

@@ -8,16 +8,20 @@ if [[ ! -d "$1" || -z "$2" ]]; then
     exit 1
 fi
 
+# convert $1 and $2 to abs paths
+SRC_DIR="$(realpath "$1" 2> /dev/null || grealpath "$1")"
+DST_DIR="$(realpath "$2" 2> /dev/null || grealpath "$2")"
+
 if ! which msgfmt > /dev/null 2>&1; then
     echo "Please install gettext"
     exit 1
 fi
 
-cd "$1"
-mkdir -p "$2"
+cd "$SRC_DIR"
+mkdir -p "$DST_DIR"
 
 for i in *; do
-    dir="$2/$i/LC_MESSAGES"
+    dir="$DST_DIR/$i/LC_MESSAGES"
     mkdir -p "$dir"
     (msgfmt --output-file="$dir/electrum.mo" "$i/electrum.po" || true)
 done