update GitHub Actions to use Go 1.18rc1

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #3335 (7b0b983) into master (fd2c345) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3335      +/-   ##
==========================================
+ Coverage   85.61%   85.62%   +0.01%     
==========================================
  Files         136      136              
  Lines        9808     9808              
==========================================
+ Hits         8397     8398       +1     
+ Misses       1036     1035       -1     
  Partials      375      375              

Impacted Files	Coverage Δ
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

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