fix: prevent installation of incompatible envier==0.5.1 (#11286)

[adamsxs]

Use of EnvVariable._cast attribute (requires envier>=0.5.2) was introduced in commit 48c2a2c, released in 2.15.1, and the requirement for ddtrace was set to ~=0.5 back in early 2024 in 00a7f00. This is generally fine but as seen in the issue #11286 it's still occasionally possible to end up with envier==0.5.1 in your environment and cause ddtrace to fail.

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[adamsxs]

System Tests appear to be failing due to an unset You must set an DD_API_KEY environment variable to run the Datadog Agent container. I noticed that field was blank in one of the build steps: https://github.com/DataDog/dd-trace-py/actions/runs/13460183428/job/37613373156?pr=12452#step:4:7
Forgive the ignorance here, first time contributing to the repo and still learning Github Actions. I'll re-base the base branch just in case, but if there's a lookup on something I should be setting please let me know.