Issue #566: don't clean upon pkgdown deployment.

[sbfnk]

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Description

This PR closes #566.

It stops cleaning the repo when deploying.

No news item/checks etc. needed as package code unaffected.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b962761) 90.92% compared to head (66759f8) 90.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #567   +/-   ##
=======================================
  Coverage   90.92%   90.92%           
=======================================
  Files          23       23           
  Lines        1399     1399           
=======================================
  Hits         1272     1272           
  Misses        127      127           

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[seabbs]

LGTM