Allow running bp2build as part of a regular build.

This is done by setting the INTEGRATED_BP2BUILD environment variable
when invoking the build.

Even though the name of the marker file insinuates that a Bazel
workspace is already created, this is not the case yet.

An issue that remains is that a .d file is not written for the marker
file so it won't be rebuilt if a .bp file changes. Fixing this requires
delicate surgery because writing the .d file is the result of delicate
interplay between Soong and Blueprint.

There are also a number of semi-related fixes:

- The name of soong.environment.{used,available} is now on the command
  line of soong_build (soong_docs is still special cased because its
  command line in the Ninja file is taken from the os.Args of
  soong_build so it's not trivial to remove the --{available,used}_env
  from it
- bp2build writes a separate soong.environment.used file
- I had to call SetAllowMissingDependencies() separately when creating
  the android.Context for bp2build so that bp2build runs in the
  integration tests (it was not obvious how not to do this)
- Fixed a number of integration tests where a command with an expected
  exit code of 1 was used as the last one in a test case, thereby
  breaking the test suite

Test: Presubmits.
Change-Id: Ibeb61c26022cf801dcb98505b4039151b3409873