legacy: fix deprecation warning about multipass --mem

[abitrolly]

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The fixes the warning.

warning: "--mem" long option will be deprecated in favour of "--memory" in a future release.Please update any scripts, etc.

[codecov-commenter]

Codecov Report

Merging #4103 (c9d386d) into main (61d519e) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main    #4103   +/-   ##
=======================================
  Coverage   94.62%   94.62%           
=======================================
  Files         643      643           
  Lines       52069    52069           
=======================================
  Hits        49270    49270           
  Misses       2799     2799           

Impacted Files	Coverage Δ
...uild_providers/multipass/test_multipass_command.py	99.63% <ø> (ø)
...l/build_providers/_multipass/_multipass_command.py	83.67% <100.00%> (ø)

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