Make sure to always create cache directory when using ICON grid files

[schlunma]

Description

Currently, the ICON on-the-fly CMORizer might fail if a user does not already have the $HOME/.esmvaltool/cache directory. The relevant mkdir is called AFTER the first usage of this cache directory. This PR fixes that.

Checklist

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[codecov]

Codecov Report

Merging #2030 (69f4efd) into main (f192e40) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2030   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files         236      236           
  Lines       12464    12464           
=======================================
  Hits        11565    11565           
  Misses        899      899           

Impacted Files	Coverage Δ
esmvalcore/cmor/_fixes/icon/_base_fixes.py	100.00% <100.00%> (ø)

[sloosvel]

I am not very familiar with the ICON cmorizer, but the changes look sensible, thanks!