Allow disabling the `isidentifier` check in `build_tarballs`

[ararslan]

Validation of the field that will determine the name of the generated JLL package is what you want 99.9% of the time. But that other 0.1% of the time is Yggdrasil/0_RootFS/PlatformSupport/build_tarballs.jl: each invocation of the script specifies a separate platform, the triplet of which gets embedded in the name. Unfortunately e.g. PlatformSupport-x86_64-unknown-freebsd13.2 is not a valid identifier, so building PlatformSupport with recent BinaryBuilder fails.

[ararslan]

It occurred to me that since #1117 removed the later name validation in favor of doing it early, adding the option to bypass validation here means that you may never validate the name. Not sure whether that's a bug or a feature of this PR but I figure it's worth noting.