Please allow setting MAX_EXECUTION_TIME in /usr/local/etc/poudriere.d/{vm_name}-make.conf on a per-port basis

[yurivict]

Prerequisites

• Have you checked for an existing issue describing your problem?
• Are you running the latest version?
• Is your ports tree recent?
• Is your FreeBSD Host on a supported release?

Describe the bug

www/chromium fails with build/timeout for a very long time now.
This is because it legitimately requires more time to build due to its size.

We need to be able to set MAX_EXECUTION_TIME_chromium=120000 (or higher) for this build to be able to complete.

Additional context

Please look at consistent failures for www/chromium here: http://beefy12.nyi.freebsd.org/jail.html?mastername=140amd64-default

[yurivict]

PR: #1155

I verified that this patch works.
By default the execution time timeout is checked every 12 minutes.