Fix detection of Rust executable detection

Before this commit, the Python code was looking for Rust executables in swh-graph's executables directory, instead of swh-provenance's

Jenkins job DPROV/gitlab-builds #168 failed in 4 min 34 sec.
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Looks like we got hit by the same issue. I fixed it in a different way in !191 (merged) but only for the tests so your solution is better.

I also found other issues that I fixed in the MR above, I guess I could rebase on these changes afterwards.

oh sorry, I didn't get the notification because I wasn't subscribed to this repo

@jenkins retry build

Jenkins job DPROV/gitlab-builds #169 succeeded in 6 min 2 sec.
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merged