fix: Update System.env syntax for Gradle 9 compatibility

[dbwiddis]

This PR updates the Gradle build files to fix compatibility with Gradle 9.

Problem

The current syntax using $System.env.VARIABLE_NAME for accessing environment variables is not compatible with Gradle 9.

This format is used in this repository for Sonatype credentials for SNAPSHOT publication. This fails with under Gradle 9.

credentials {
    username = "$System.env.SONATYPE_USERNAME"
    password = "$System.env.SONATYPE_PASSWORD"
}

See example failed workflow here.

Solution

Update to use the System.getenv() method instead:

credentials {
    username = System.getenv("SONATYPE_USERNAME")
    password = System.getenv("SONATYPE_PASSWORD")
}

Example fix which restored successful snapshots: opensearch-project/opensearch-remote-metadata-sdk#245

[github-actions]

❌ Gradle check result for b6b8f49: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for b6b8f49: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.79%. Comparing base (c9eb7fb) to head (b6b8f49).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19119      +/-   ##
============================================
- Coverage     73.00%   72.79%   -0.21%     
+ Complexity    69518    69405     -113     
============================================
  Files          5648     5648              
  Lines        319230   319230              
  Branches      46174    46174              
============================================
- Hits         233039   232397     -642     
- Misses        67346    68039     +693     
+ Partials      18845    18794      -51     

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