Move UV install to after the ENV is set

[thomasjpfan]

Why are the changes needed?

On master, the UV install command runs before the ENV is set.

What changes were proposed in this pull request?

This PR moves ENV before the UV command runs.

How was this patch tested?

I ran pytest with _FLYTEKIT_TEST_DEFAULT_BUILDER=1.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.06%. Comparing base (69445ff) to head (31ef8a1).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2681      +/-   ##
==========================================
- Coverage   79.24%   73.06%   -6.18%     
==========================================
  Files         196      207      +11     
  Lines       19785    19921     +136     
  Branches     4008     4006       -2     
==========================================
- Hits        15678    14555    -1123     
- Misses       3407     4692    +1285     
+ Partials      700      674      -26     

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