[roll] Roll infra/recipes [rust] Trim whitespace from env variables

This fixes a bug we have had since fxr/1365276 that caused us to
pass incorrect command-line arguments to x.py in the rust toolchain
recipe.

Previously, we passed:
  '--test-args',
  '--target-rustcflags',
  '--test-args',
  '-Cforce-unwind-tables=yes',
  '--test-args',
  '--remote-test-client',

After that change we passed:
  '--test-args',
  '--target-rustcflags',
  '--test-args',
  '-Cforce-unwind-tables=yes',
  '--test-args',
  '--target-rustcflags',
  '--test-args',
  '',
  '--test-args',
  '--remote-test-client',

This was due to us sometimes appending empty string to the list of
target flags. These flags were processed in a loop:

    test_args += [
        arg
        for flag in target_flags
        for arg in ["--test-args", "--target-rustcflags", "--test-args", flag]
    ]

.. which added extra command-line args that should not have been
there. This had the effect of x.py passing "--remote-test-client" down
to rustc, which did not know how to handle it.

This CL trims the environment of whitespace between converting them
into arguments to be passed to x.py.

Original-Bug: b/446708220
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/recipes/+/1476408
Original-Revision: 6541d9b52ace31a1be24ef11ae4fcc1b875f8511
GitOrigin-RevId: e8a2c7be6905ce3c464802a67afa2ec3ac25931b
Change-Id: Ib6eca7a8e5b0b474965a0a2b96304562d2d2bc41