fix: provide container plugin path to memcheck tests

[robbycochran]

Description

Set ROX_COLLECTOR_CONTAINER_PLUGIN_PATH for the separately registered memcheck_ CTest entries, just as it is set for the normal unit tests.

Targets collector-container-plugin (#3939); this is a sibling of #3940, not dependent on the validator changes. One-line test configuration fix, no production changes.

Failure

Both existing Valgrind unit-test jobs fail SystemInspectorServiceTest.FilterEvent before loading the plugin:

Expected: (plugin_path) != (nullptr), actual: NULL vs (nullptr)

CTest's environment property is attached to the normal test, not automatically inherited by the separately registered memcheck_SystemInspectorServiceTest.

Failing jobs: AMD64, ARM64. Both report 32/33 tests passing; the sole failed entry is memcheck_SystemInspectorServiceTest. This addresses the missing-environment assertion, not the separate library diagnostics in Valgrind output.

Validation

• Fresh Linux ARM64 builder configuration with -DUSE_VALGRIND=ON -DCMAKE_BUILD_TYPE=Debug succeeds.
• ctest --show-only=json-v1 confirms identical plugin-path environment values for normal and memcheck SystemInspectorServiceTest entries.
• git diff --check passes.
• Full test execution has not been rerun locally; CI will validate execution.

skip-integration-tests skips the broad VM/Kubernetes matrix for this test-configuration-only fix; existing unit-test jobs remain enabled.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.33%. Comparing base (249fe75) to head (853156e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                     Coverage Diff                     @@
##           collector-container-plugin    #3941   +/-   ##
===========================================================
  Coverage                       27.33%   27.33%           
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  Files                              94       94           
  Lines                            5399     5399           
  Branches                         2538     2538           
===========================================================
  Hits                             1476     1476           
  Misses                           3197     3197           
  Partials                          726      726           

Flag	Coverage Δ
collector-unit-tests	27.33% <ø> (ø)

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