CI: cancel old PR builds when new commits are pushed to a PR branch

[DilumAluthge]

This leads to better resource utilization.

[codecov-commenter]

Codecov Report

Merging #414 (cf2a0f7) into master (147c8c2) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files          36       36           
  Lines        1760     1760           
=======================================
  Hits         1680     1680           
  Misses         80       80           

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[DilumAluthge]

@omus Two CI jobs failed on Julia nightly with segfaults. Is this a known issue?

[DilumAluthge]

Alright, after a retry, all jobs are passing.

@omus This is ready to review.

[DilumAluthge]

Bump @omus

[omus]

Sorry for the slow response. Looking at this now.