Add option to DartResourceRetriever to search from environment variable DART_DATA_PATH

[jslee02]

This PR adds a fallback option to DartResourceRetriever to use an environment variable to search DART data files. The existing preprocessor, DART_DATA_GLOBAL_PATH, doesn't work if the package manager uses temporary install directory (e.g., Launchpad PPA and Homebrew).

This should resolve #1091 and dartsim/homebrew-dart#54.

---

Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md

[codecov]

Codecov Report

Merging #1095 into release-6.6 will increase coverage by 0.02%.
The diff coverage is 93.75%.

@@               Coverage Diff               @@
##           release-6.6    #1095      +/-   ##
===============================================
+ Coverage        56.59%   56.61%   +0.02%     
===============================================
  Files              320      320              
  Lines            24524    24539      +15     
===============================================
+ Hits             13879    13893      +14     
- Misses           10645    10646       +1

Impacted Files	Coverage Δ
dart/utils/DartResourceRetriever.hpp	100% <100%> (ø)	⬆️
dart/utils/DartResourceRetriever.cpp	89.33% <93.33%> (+1%)	⬆️