"spack build-env" searches env for relevant spec (#21642)

If you install packages using spack install in an environment with
complex spec constraints, and the install fails, you may want to
test out the build using spack build-env; one issue (particularly
if you use concretize: together) is that it may be hard to pass
the appropriate spec that matches what the environment is
attempting to install.

This updates the build-env command to default to pulling a matching
spec from the environment rather than concretizing what the user
provides on the command line independently.

This makes a similar change to spack cd.

If the user-provided spec matches multiple specs in the environment,
then these commands will now report an error and display all
matching specs (to help the user specify).

Co-authored-by: Gregory Becker <becker33@llnl.gov>

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