cli: Use os.availableParallelism() instead of os.cpus().length (#46247)

Node has this function (and has since v18.14, apparently) that's
specifically intended to give us the number we want. We should use it.

Author: anomiex

tools/cli/commands/phan.js

@@ -67,7 +67,7 @@ export async function builder( yargs ) {
 		.option( 'concurrency', {
 			type: 'number',
 			description: 'Maximum number of phan tasks to run at once.',
-			default: os.cpus().length,
+			default: os.availableParallelism(),
 			coerce: coerceConcurrency,
 		} )
 		.option( 'format', {

tools/cli/commands/test.js

@@ -48,7 +48,7 @@ export async function builder( yargs ) {
 		.option( 'concurrency', {
 			type: 'number',
 			description: 'Maximum number of test tasks to run at once. Ignored with `--verbose`.',
-			default: os.cpus().length,
+			default: os.availableParallelism(),
 			coerce: coerceConcurrency,
 		} )
 		.option( 'no-html', {