Add gradle wrapper cache to continuous monitoring workflow

[willarmiros]

Issue #, if available:

Description of changes:
Attempt to fix transient monitoring workflow failures like https://github.com/aws/aws-xray-sdk-java/actions/runs/941160994 which timeout in downloading the Gradle binary. I do not believe believe that caching the gradle binary will have any impact on the accuracy of this for smoke testing the SDK's availability/functionality, but please confirm this understanding @anuraaga.

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

[codecov-commenter]

Codecov Report

Merging #287 (eadef75) into master (fd3cc7b) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #287   +/-   ##
=========================================
  Coverage     58.86%   58.86%           
  Complexity     1202     1202           
=========================================
  Files           131      131           
  Lines          5056     5056           
  Branches        588      588           
=========================================
  Hits           2976     2976           
  Misses         1807     1807           
  Partials        273      273           

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