updatekeys: Make file path absolute

[mhyllander]

"sops updatekeys" is not working the same as when encrypting a file. The
reason is that for "sops --encrypt", the file path is made absolute before
it is compared with the path_regex in the config file. This is not done for
"sops updatekeys", therefore it does not match the correct entry in the
config file when updating keys.

[codecov-commenter]

Codecov Report

Merging #682 into develop will increase coverage by 0.15%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #682      +/-   ##
===========================================
+ Coverage    38.08%   38.23%   +0.15%     
===========================================
  Files           23       23              
  Lines         3329     3329              
===========================================
+ Hits          1268     1273       +5     
+ Misses        1931     1927       -4     
+ Partials       130      129       -1     

Impacted Files	Coverage Δ
stores/flatten.go	91.52% <0.00%> (+4.23%)	⬆️

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