Fix local webdriver discovery via PATH

[j-devel]

Fixes the following bug (minor regression after eeebec0)

Describe the Bug

Local webdriver discovery via PATH is not working.

Steps to Reproduce

$ cd crates/web-sys
$ which geckodriver  # make sure geckodriver is installed and in PATH
/usr/local/bin/geckodriver
$ unset GECKODRIVER_REMOTE  # make sure not remotely using geckodriver via env vars
$ unset GECKODRIVER  # make sure not using geckodriver via env vars
$ cargo test --target wasm32-unknown-unknown --all-features

Expected Behavior

...
Running headless tests in Firefox on `http://127.0.0.1:55305/`
...
test result: ok. 56 passed; 0 failed; 0 ignored

Actual Behavior

/usr/local/bin/geckodriver is incorrectly resolved to "/user/local/bin":

...
error: failed to spawn "/usr/local/bin" binary
	caused by: Permission denied (os error 13)
error: test failed, to rerun pass '--test wasm'

[alexcrichton]

Oops, thanks!