Add errors="backslashreplace" to run_with_env_secrets.py

[lucylq]

See: https://fb.workplace.com/groups/4571909969591489/permalink/7818394354943018/

ExecuTorch CI was running into an issue where non-utf8 print output caused tests to crash. This was resolved by removing the printing in pytorch/executorch#4268.

This PR updates the error handling to use backslashreplace, which should still show us the error/printing, but without crashing the test.

From https://docs.python.org/3.9/library/io.html#io.TextIOWrapper:

'backslashreplace' causes malformed data to be replaced by a backslashed escape sequence. 

From https://docs.python.org/3/library/subprocess.html#frequently-used-arguments:

If encoding or errors are specified, or text (also known as universal_newlines) is true, the file objects stdin, stdout and stderr will be opened in text mode using the encoding and errors specified in the call or the defaults for io.TextIOWrapper.

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[lucylq]

test-secrets-filter-var tests are failing, though they seem to fail on an empty PR too, see #5458. Also looks like they're run sporadically, from HUD: https://hud.pytorch.org/hud/pytorch/test-infra/main/1?per_page=50

Do these tests require specific permissions?

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[huydhn]

run_with_env_secrets script is one of those few workflow whose tests would fail if the PR is a forked PR, so let me quickly double check this before landing

[huydhn]

All tests pass on my end https://github.com/pytorch/test-infra/pull/5492/checks, so I guess this is good to go.