EXEMPT_ECOSYSTEMS accepts invalid ecosystem names without validation

[zkoppert]

Bug

EXEMPT_ECOSYSTEMS does not validate its values against the list of supported package ecosystems. Any string is silently accepted, so a typo like "docekr" instead of "docker" would be ignored without error, leading to unexpected behavior (the ecosystem wouldn't actually be exempted).

Expected behavior

EXEMPT_ECOSYSTEMS should validate each ecosystem against the same allowed list used by REPO_SPECIFIC_EXEMPTIONS:

bundler, cargo, composer, docker, github-actions, gomod, mix, npm, nuget, pip, terraform

An unrecognized value should raise a ValueError, consistent with how parse_repo_specific_exemptions handles invalid ecosystems.

Current behavior

https://github.com/github-community-projects/evergreen/blob/main/env.py#L241-L245

exempt_ecosystems_list = [
    ecosystem.lower().strip() for ecosystem in exempt_ecosystems.split(",")
]

Values are lowercased and stripped, but never checked against the supported ecosystem list.

Additional context

Discovered while reviewing PR #488, which fixed a related whitespace-trimming bug in REPO_SPECIFIC_EXEMPTIONS. That parser validates ecosystems; this one does not.