Prepend the Pepys path to the PATH variable, rather than appending it

[robintw]

🧰 Issue

Fixes #1107.

🚀 Overview:

Changes the installation script to prepend the Pepys path to the PATH environment variable, so it takes priority over other entries on the path.

🤔 Reason:

So the correct version of Pepys is used.

🔨Work carried out:

• Modify install pepys script
• Tests pass

Confirmations

• I have chosen reviewers for my PR.
• I have chosen an appropriate label for the PR, adding interactive_review if reviewers will need to see UI
• I have extended/updated the documentation in \docs folder
• Any database content changes (Create, Edit, Delete) are recorded in the Log/Changes tables
• Any database schema changes are implemented via alembic revision transitions
• I have completed the mandatory sections of this document.
• I have deleted any unused sections.

[robintw]

In this PR I've done a 'quick fix' that prepends the Pepys path rather than appends it. It might be possible to look for other possible Pepys paths and remove those, but that's going to be a little tricky and take a while - and I'm not sure whether it's actually the best idea. We can discuss that going forward, but for now this should at least get things working.

[codecov]

Codecov Report

Merging #1108 (1c7bc51) into develop (4b1954b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1108   +/-   ##
========================================
  Coverage    80.08%   80.08%           
========================================
  Files          110      110           
  Lines        11896    11896           
========================================
  Hits          9527     9527           
  Misses        2369     2369           

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