Make lint_pkgbuild opt-in via MAKEPKG_LINT_PKGBUILD=1

[lazka]

Cygwin bash is struggling with the thousands of commands that get invoked when lint_pkgbuild is run. This especially is annoying for split packages because the calls get multiplied by the package count.

For example when running --printsrcinfo:

msys/brotli (4 packages)

• 39.8 seconds with linting
• 7 seconds without

msys/tar (1 package)

• 10.2 seconds with linting
• 2 seconds without

This changes the default to not lint the PKGBUILD and adds a MAKEPKG_LINT_PKGBUILD env var that can be used to enable linting. We can for example enable this in CI.

[cbrt64]

Holy mackerel, that's faster! I assumed it was unavoidable fork delays, or I would have complained a while ago. Instant makepkg startup instead of waiting ~20 s. Thank you, sir.

[lazka]

I had this lying around way too long and patching it locally.... sry.. but better late then never.