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 69.80%. Comparing base (fedf867) to head (f987e66).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (69.80%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #821   +/-   ##
=======================================
  Coverage   69.80%   69.80%           
=======================================
  Files          38       38           
  Lines        1729     1729           
  Branches      156      156           
=======================================
  Hits         1207     1207           
  Misses        431      431           
  Partials       91       91           

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