@actions/exec: convert to ESM module

__tests__/scripts/stdlineoutput.cjs

@@ -0,0 +1,5 @@
+//Default highWaterMark for readable stream buffers us 64K (2^16)
+//so we go over that to get more than a buffer's worth
+const os = require('os')
+
+process.stdout.write('a'.repeat(2**16 + 1) + os.EOL);