parsing ambiguity in in_workunit! macro

[tdyas]

Parsing ambiguity seen while trying to use as operator at the end of closure passed to in_workunit! macro:

error: local ambiguity when calling macro `in_workunit`: multiple parsing options: built-in NTs tt ('body') or 1 other option.
   --> process_execution/src/local.rs:546:43
    |
546 |         Ok(exclusive_spawn) as Result<bool, String>
    |                                           ^

[huonw]

#13483 / 30640b2#diff-230c46e3890a73dee860295c27128ef3aadc239d2db0f6bcd1ac67db038385efL851 seemed to change this from using tt to using expr, which likely resolves the ambiguity.