Allow debug_dump_gcs to be specified with other XLA_FLAGS

[jonb377]

Fixes / Features

• debug-dump-gcs doesn't need to be exclusive with environment-specified XLA_FLAGS.

Testing / Documentation

Testing details:

• xpk workload create ... --debug-dump-gcs gs://foo/bar --env XLA_FLAGS=--xla_dump_to=/foo/bar => ValueError: Conflict: --xla_dump_to flag defined by both --debug_dump_gcs and XLA_FLAGS in container environment. Please choose one way to define.

• xpk workload create ... --debug-dump-gcs gs://foo/bar --env XLA_FLAGS=--xla_foo=bar => workload created with XLA_FLAGS --xla_foo=bar --xla_dump_to=/tmp/xla_dump/

• xpk workload create ... --debug-dump-gcs gs://foo/bar => workload created with XLA_FLAGS --xla_dump_to=/tmp/xla_dump/ (note the leading space, but XLA has no problem parsing).

• [ y ] Tests pass

• [ y ] Appropriate changes to documentation are included in the PR