fix: Do not use absolute paths to the postprocessors

[lens0021]

If any postprocessor is not installed at the exact path(ex: ~/.local/bin/bshchk), the cargo test tests::postprocessor::default_ok fails:

running 1 test
test tests::postprocessor::default_ok ... FAILED

failures:

---- tests::postprocessor::default_ok stdout ----
thread 'tests::postprocessor::default_ok' panicked at src/tests/postprocessor.rs:21:5:
These commands have to be in $PATH for this test to pass: bshchk
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    tests::postprocessor::default_ok

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 222 filtered out; finished in 0.00s

This PR omits /usr/bin/.

If program is not an absolute path, the PATH will be searched in an OS-defined way.
-- Struct std::process::Command

[Mte90]

I think that change was implemented for Windows, what do you think @hdwalters @b1ek?

I had the same issue but I resolved with a symlink locally.

[hdwalters]

I think that change was implemented for Windows, what do you think @hdwalters @b1ek?

I had the same issue but I resolved with a symlink locally.

The hard coded path was added by @b1ek as part of the recent postprocessor update. The change in this PR looks ok to me.