Update to AiiDA v1.6.4 and remove CI fix

[CasperWA]

Closes #855

[ml-evs]

Assuming the CI passes! (can re-review after lunch if not)

[codecov]

Codecov Report

Merging #857 (4964943) into master (a4d3424) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #857   +/-   ##
=======================================
  Coverage   92.64%   92.64%           
=======================================
  Files          67       67           
  Lines        3724     3724           
=======================================
  Hits         3450     3450           
  Misses        274      274           

Flag	Coverage Δ
project	92.64% <ø> (ø)
validator	92.64% <ø> (ø)

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[JPBergsma]

Seems good to me.