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 73.55%. Comparing base (2b76e3c) to head (4c04dea).
⚠️ Report is 92 commits behind head on main.

❌ Your project status has failed because the head coverage (73.55%) 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     #630      +/-   ##
============================================
- Coverage     81.78%   73.55%   -8.24%     
- Complexity      193      376     +183     
============================================
  Files            11       21      +10     
  Lines           961     2004    +1043     
  Branches        137      291     +154     
============================================
+ Hits            786     1474     +688     
- Misses          121      430     +309     
- Partials         54      100      +46     

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