CI: update rust nightly and packed_simd

[ritchie46]

Which issue does this PR close?

This updates the nightly version in CI to a recent version. This should fix some problems that arise when compiling simd on latest nightly rustc version.

Closes #517

[codecov-commenter]

Codecov Report

Merging #525 (aa77150) into master (21d69ca) will not change coverage.
The diff coverage is n/a.

❗ Current head aa77150 differs from pull request most recent head 1ea1c26. Consider uploading reports for the commit 1ea1c26 to get more accurate results

@@           Coverage Diff           @@
##           master     #525   +/-   ##
=======================================
  Coverage   82.54%   82.54%           
=======================================
  Files         167      167           
  Lines       45956    45956           
=======================================
  Hits        37933    37933           
  Misses       8023     8023           

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[alamb]

Thanks @ritchie46