Makefile: Restore vendor directory after dep-validate

[aaronlehmann]

Currently, this target makes changes to the vendor directory, which may
overwrite local changes or cause build errors. It's better to restore
the original vendor directory after running the check.

[dmcgowan]

LGTM

[codecov]

Codecov Report

Merging #2218 into master will decrease coverage by 10.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2218       +/-   ##
===========================================
- Coverage   61.74%   51.72%   -10.02%     
===========================================
  Files         125      125               
  Lines       11452    11452               
===========================================
- Hits         7071     5924     -1147     
- Misses       3495     4782     +1287     
+ Partials      886      746      -140

Impacted Files	Coverage Δ
registry/storage/driver/gcs/gcs.go	0.4% <0%> (-68.94%)	⬇️
registry/storage/driver/oss/oss.go	0.56% <0%> (-57.23%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.58% <0%> (-56.31%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.5% <0%> (-50.76%)	⬇️
registry/client/transport/transport.go	69.69% <0%> (-9.1%)	⬇️

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