flaky tests in GitHub actions

[willmurphyscode]

The unit tests for this repo sometimes fail with an error like this:

spawn ETXTBSY

      at ToolRunner.<anonymous> (node_modules/@actions/exec/src/toolrunner.ts:443:24)
      at node_modules/@actions/exec/lib/toolrunner.js:27:71
      at Object.<anonymous>.__awaiter (node_modules/@actions/exec/lib/toolrunner.js:23:12)
      at node_modules/@actions/exec/src/toolrunner.ts:419:58
      at ToolRunner.<anonymous> (node_modules/@actions/exec/src/toolrunner.ts:419:12)
      at fulfilled (node_modules/@actions/exec/lib/toolrunner.js:24:58)

(link)

I believe this is because the tests run simultaneously, but runGrype is not threadsafe.

scan-action/index.js

Lines 31 to 35 in 52d017b

	let grypePath = cache.find(grypeBinary, version);
	if (!grypePath) {
	// Not found, install it
	grypePath = await downloadGrype(version);
	}

This has a race condition, since whatever is present in the cache may be changed by one test while another test is checking it. I've also seen ENOENT in test runs.

[willmurphyscode]

We might be able to get away with just running one test at a time as a cheap way to fix this:

With maxConcurrency = 1, we see npm run test 11.77s user 6.00s system 33% cpu 53.132 total. But nearly 50 seconds of that is downloading the db (which only happens once regardless of test parallelism).

I'll see if maxConcurrency = 1 fixes this.