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 73.05%. Comparing base (b475ff7) to head (5a0481c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5577      +/-   ##
==========================================
+ Coverage   73.02%   73.05%   +0.03%     
==========================================
  Files         408      408              
  Lines       25259    25259              
  Branches     3842     3842              
==========================================
+ Hits        18445    18454       +9     
+ Misses       4945     4937       -8     
+ Partials     1869     1868       -1     

see 6 files with indirect coverage changes

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