Update vswhere.py

[MattStephanson]

Fixes the following error that I encountered:

Action failed: prelude//toolchains/msvc:msvc_tools (vswhere)
Local command returned non-zero exit code 1
Reproduce locally: `env -- "BUCK_SCRATCH_PATH=buck-out\\v2\\tmp\\prelude\\5540ca4c71e1230f\\toolchains\\msvc\\__msvc_too ...<omitted>... " "--rc=buck-out\\v2\\gen\\prelude\\5540ca4c71e1230f\\toolchains\\msvc\\__msvc_tools__\\rc.exe.json" (run `buck2 log what-failed` to get the full command)`
stdout:
stderr:
Traceback (most recent call last):
  File "C:\Users\mstephan\source\buck2-test\buck-out\v2\gen\prelude\5540ca4c71e1230f\toolchains\msvc\__vswhere__\vswhere.py", line 331, in <module>
    main()
  File "C:\Users\mstephan\source\buck2-test\buck-out\v2\gen\prelude\5540ca4c71e1230f\toolchains\msvc\__vswhere__\vswhere.py", line 312, in main
    rc_exe = find_in_path("rc.exe", optional=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: find_in_path() got an unexpected keyword argument 'optional'

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