Performance tests: Install locked dependencies without resolution

When running the performance tests we're installing our `npm`
dependencies using `npm install`. This not only takes longer to run than
`npm ci`, but also results in non-deterministic builds which could
flavor the results of our tests.

In this patch we're using `npm ci` instead which should result in more
consistent test runs with less setup overhead.

bin/plugin/commands/performance.js

@@ -161,10 +161,7 @@ async function setUpGitBranch( branch, environmentDirectory ) {
 	await git.checkoutRemoteBranch( environmentDirectory, branch );
 
 	log( '        >> Building the ' + formats.success( branch ) + ' branch' );
-	await runShellScript(
-		'npm install && npm run build',
-		environmentDirectory
-	);
+	await runShellScript( 'npm ci && npm run build', environmentDirectory );
 }
 
 /**
@@ -235,7 +232,7 @@ async function runPerformanceTests( branches, options ) {
 	}
 	log( '    >> Installing dependencies and building packages' );
 	await runShellScript(
-		'npm install && npm run build:packages',
+		'npm ci && npm run build:packages',
 		performanceTestDirectory
 	);
 	log( '    >> Creating the environment folders' );