Fix v85.go

[zeripath]

In trying to create migration tests for MSSQL I've noticed that the v85 sets the UNIQUE constraint too early.

This PR fixes this by asserting the creation of the UNIQUE index later.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-io]

Codecov Report

Merging #6851 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6851      +/-   ##
==========================================
- Coverage   41.18%   41.17%   -0.02%     
==========================================
  Files         423      423              
  Lines       58399    58416      +17     
==========================================
- Hits        24052    24051       -1     
- Misses      31169    31185      +16     
- Partials     3178     3180       +2

Impacted Files	Coverage Δ
models/migrations/v85.go	0% <0%> (ø)	⬆️
modules/log/file.go	75.52% <0%> (-2.1%)	⬇️
models/unit.go	67.56% <0%> (+5.4%)	⬆️

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