Use pull_request_target event for branch protection CI

[ewels]

Closes #765

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #766 (4e7437a) into dev (3c8730c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #766   +/-   ##
=======================================
  Coverage   83.03%   83.03%           
=======================================
  Files          13       13           
  Lines        2753     2753           
=======================================
  Hits         2286     2286           
  Misses        467      467           

Impacted Files	Coverage Δ
nf_core/lint.py	91.13% <100.00%> (ø)

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[abhi18av]

After going through the contents of #765 , I'm looking forward to see this in action!

[ewels]

Nice, it worked! nf-core/testpipeline#3