build-sys: meson: Give ourselves a little more timeout headroom

We're seeing CI failures due to once-test sometimes taking long. Let's
give ourselves a little more space per test (2 min -> 5 min) to avoid
this.
2 jobs for master in 4 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #511756
build-autotools

00:04:34

passed #511757
build-meson

00:01:12