Add "pmbootstrap build --no-depends"

Two commits:

Cosmetic: remove confusing init_buildenv comments

Build dependencies don't necessarily get installed for the host arch, that depends on the build method ("native" or not).

Add pmbootstrap build --no-depends

Aborts the build if any dependencies would have to be build first. This is useful for build.postmarketos.org, because we want to build exactly one package in one build job. If dependencies would need to be built, we made a mistake earlier, and not aborting the build makes it harder to find that orginal mistake.

How to test:

$ pmbootstrap zap -p # careful, deletes all your local packages!
$ pmbootstrap -mp="" build --no-depends hello-world-wrapper
[10:02:38] ERROR: Missing binary package for dependency 'hello-world' of 'hello-world-wrapper', but pmbootstrap won't build any depends since it was started with --no-depends.
[10:02:38] See also: <https://postmarketos.org/troubleshooting>
Run 'pmbootstrap log' for details.