Fix coverage executable substitution in template (#1297)

Looks like the change in
d6b7f13
forgot to replace the `%executable%` reference for running coverage.

Author: sebastianv1

tools/xctest_runner/xctest_runner.sh.template

@@ -88,7 +88,7 @@ xcrun llvm-cov \
   -instr-profile "$profdata" \
   -ignore-filename-regex='.*external/.+' \
   -path-equivalence="$ROOT",. \
-  "%executable%" \
+  "$executable_path" \
   @"$COVERAGE_MANIFEST" \
   > "$COVERAGE_OUTPUT_FILE" \
   2> "$error_file" \