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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.61%. Comparing base (17e56f0) to head (5d14dc1).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1554      +/-   ##
============================================
+ Coverage     81.59%   81.61%   +0.01%     
- Complexity     6039     6042       +3     
============================================
  Files           536      536              
  Lines         24427    24427              
  Branches       2494     2494              
============================================
+ Hits          19932    19935       +3     
- Misses         3260     3261       +1     
+ Partials       1235     1231       -4     

Flag	Coverage Δ
plugin	81.61% <ø> (+0.01%)	⬆️

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see 11 files with indirect coverage changes

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[kaituo]

@dbwiddis Can you add changelog?