Bash when root: set PS4 in bash-helper instead of the environment.

[Orycterope]

See #331

The first commit is just there to prevent the line PS4=... in the helper to be printed when executed, which is inconvenient. There was no need to source the helper with -x, provided it sets -x anyway.

[codecov[bot]]

Codecov Report

Merging #332 into master will decrease coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
- Coverage   70.36%   70.30%   -0.06%     
==========================================
  Files          57       57              
  Lines        3718     3711       -7     
==========================================
- Hits         2616     2609       -7     
  Misses       1102     1102              

Impacted Files	Coverage Δ
src/engines/bash-engine.cc	89.29% <ø> (-0.25%)	:arrow_down:

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[coveralls]

Coverage decreased (-0.06%) to 70.305% when pulling 85e7d80417debe4d354046f87f275d3f8de06360 on Orycterope:master into 4e4c8ad86865e60b07cd4bb457a9f207e950ecea on SimonKagstrom:master.

[Orycterope]

Hummm ... I don't get how removing lines can decrease the coverage percentage 🤔

I hope this is not important for this PR to be merged.

[SimonKagstrom]

The coverage decrease sounds like a bug in coveralls/codecov, so don't worry about that :-)

Anyway, good stuff and a nice solution, thanks a lot!