Add env variable SOTO_CROSS_COMPILE to enable cross compile of package

[adam-fowler]

This is a complete hack, but without https://bugs.swift.org/browse/SR-13761 being fixed soto-core requires the if statement at the base of the Package.swift. This causes cross compilation to fail as the Package.swift is read by macOS and the switch on which OS is being run fails.

[codecov[bot]]

Codecov Report

Merging #403 (438f64e) into main (78e03eb) will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
+ Coverage   76.70%   76.74%   +0.03%     
==========================================
  Files          63       63              
  Lines        5014     5014              
==========================================
+ Hits         3846     3848       +2     
+ Misses       1168     1166       -2     

Impacted Files	Coverage Δ
...toCore/Credential/DeferredCredentialProvider.swift	77.77% <0.00%> (-3.71%)	:arrow_down:
Sources/SotoCore/Credential/ConfigFileLoader.swift	99.28% <0.00%> (+0.71%)	:arrow_up:
...toCore/Credential/MetaDataCredentialProvider.swift	95.78% <0.00%> (+1.05%)	:arrow_up:
...toCore/Credential/CredentialProviderSelector.swift	100.00% <0.00%> (+6.66%)	:arrow_up:

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[adam-fowler]

Closing this as this is resolved on the v7.0 branch