fix: fix the lookup check for registry-config option

[lizMSFT]

This bug was introduced in PR #432.
fs.Lookup("config") != nil was intended to check whether there was an existing config to avoid repeatedly setting the config value. However, we are not able to set the config value if fs.Lookup("config") returns nil.
This PR changed the check to if fs.Lookup("config") == nil {}

Signed-off-by: Zoey Li zoeyli@microsoft.com

[codecov-commenter]

Codecov Report

Merging #505 (6b78aad) into main (a4da0a3) will increase coverage by 0.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
+ Coverage   71.90%   72.68%   +0.77%     
==========================================
  Files           9        9              
  Lines         388      388              
==========================================
+ Hits          279      282       +3     
+ Misses         87       85       -2     
+ Partials       22       21       -1     

Impacted Files	Coverage Δ
cmd/oras/internal/option/remote.go	61.90% <100.00%> (+2.85%)	⬆️

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[shizhMSFT]

LGTM