tests: quote PATH in external-builders test heredoc

The external-builders test expands `$PATH` into a heredoc without quotes,
so any `PATH` entry containing spaces causes bash to parse the line as a
command instead of an assignment, failing the test.

tests/functional/external-builders.sh

@@ -23,7 +23,7 @@ external_builder="$TEST_ROOT/external-builder.sh"
 cat > "$external_builder" <<EOF
 #! $SHELL -e
 
-PATH=$PATH
+PATH=${PATH@Q}
 
 [[ "\$1" = bla ]]