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 70.01%. Comparing base (9cb092f) to head (600a265).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #841      +/-   ##
============================================
- Coverage     70.88%   70.01%   -0.87%     
- Complexity      421      489      +68     
============================================
  Files            49       54       +5     
  Lines          3125     3518     +393     
  Branches        194      228      +34     
============================================
+ Hits           2215     2463     +248     
- Misses          785      911     +126     
- Partials        125      144      +19     

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