[native_assets_builder] Make hook environment configurable

[dcharkes]

Make the environment used to invoke hooks configurable.

The flutter roll is failing currently, and I suspect it has to do with our change to tie down the environment variables. The execution of the dart executable fails. #1847

We can't easily troubleshoot which env key is missing currently, because the environment is not configurable from the embedder.

Since we'd like the embedder to be able to configure the environment anyways (e.g. Flutter might want to pass in ANDROID_HOME but Dart not), make the change now.

Hopefully, we can then trouble-shoot what's going on. (At the very least we can try passing the full environment in flutter_tools to see if that fixes the issue.)

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:native_assets_builder	0.10.1	ready to publish	native_assets_builder-v0.10.1
package:ffi	2.1.3	already published at pub.dev
package:ffigen	16.0.1-wip	WIP (no publish necessary)
package:jni	0.13.0	already published at pub.dev
package:jnigen	0.13.0	already published at pub.dev
package:native_assets_cli	0.10.0	already published at pub.dev
package:native_toolchain_c	0.7.0	already published at pub.dev
package:objective_c	4.0.1-wip	WIP (no publish necessary)
package:swift2objc	0.0.1-wip	WIP (no publish necessary)
package:swiftgen	0.0.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[coveralls]

coverage: 87.527% (+0.001%) from 87.526%
when pulling 3ca37a6 on pass-in-env-hook
into 0495080 on main.