expose the build environment

[dvirtz]

Discussed in #1256

^{Originally posted by dvirtz February 12, 2024}
The build environment (the one named <project-name>-build) doesn't behave like the other environments.

• you can't select it with the -e switch or the env commands
• it doesn't respect the dirs.env config

This is problematic because sometimes I need to debug build errors and I want to run a shell from the build environment.
Or if want to start from scratch I delete the environment and it's more convenient if it's all under the dirs.env directory.

[ofek]

resolved by #1602