Set GOCACHE under OUT_DIR

[justsmth]

Issues

Addresses:

• All version of aws-lc-fips-sys fail to build docs on docs.rs #600

Context

• The documentation build for aws-lc-fips-sys on docs.rs is failing. The build log indicates that it's due to the Go compiler being unable to initialize its cache directory: https://docs.rs/crate/aws-lc-fips-sys/0.12.14/builds/1543179

...
[INFO] [stderr]   --- stderr
[INFO] [stderr]   failed to initialize build cache at /.cache/go-build: mkdir /.cache: permission denied
...

• The GOCACHE environment variable determines the location for this directory.

Description of changes:

• Sets the GOCACHE environment variable to be beneath ${OUT_DIR}, in which the Go compiler should have permission to create its .cache directory.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.69%. Comparing base (c358484) to head (4ccf2d7).
Report is 116 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #603      +/-   ##
==========================================
- Coverage   95.80%   92.69%   -3.11%     
==========================================
  Files          61       67       +6     
  Lines        8143     9772    +1629     
  Branches        0     9772    +9772     
==========================================
+ Hits         7801     9058    +1257     
- Misses        342      429      +87     
- Partials        0      285     +285     

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[skmcgrail]

nit: Ideally if the user has this environment variable set to something we don't override it.