Don't accidentally ignore all output in tests/run-make/coverage-reports diffs

Because the literal pipe `|` character was not escaped, these regexes ended up
accidentally ignoring every line in the coverage report output, so the tests
would not fail even if the output was wrong.

Author: Zalathar

tests/run-make/coverage-reports/Makefile

@@ -174,7 +174,7 @@ else
 	# files are redundant, so there is no need to generate `expected_*.json` files or
 	# compare actual JSON results.)
 
-	$(DIFF) --ignore-matching-lines='^  | .*::<.*>.*:$$' --ignore-matching-lines='^  | <.*>::.*:$$' \
+	$(DIFF) --ignore-matching-lines='^  \| .*::<.*>.*:$$' --ignore-matching-lines='^  \| <.*>::.*:$$' \
 		expected_show_coverage.$@.txt "$(TMPDIR)"/actual_show_coverage.$@.txt || \
 		( grep -q '^\/\/ ignore-llvm-cov-show-diffs' $(SOURCEDIR)/$@.rs && \
 			>&2 echo 'diff failed, but suppressed with `// ignore-llvm-cov-show-diffs` in $(SOURCEDIR)/$@.rs' \