fix: bugs for skaffold.yaml files read in via stdin

[aaron-prindle]

This fix modifies the behavior for 2 pieces of how skaffold.yaml's read in via stdin work:

1. Makes it so that the SourceFile field for a SkaffoldConfigEntry is - for files read in via stdin instead of /abs/path/skaffold.yaml/- which is what the value is currently.
2. Makes it so that util.ReadConfiguration is idempotent for skaffold.yaml files read in via stdin by storing the []byte from os.Stdin that are read in. Currently once the Reader that is os.Stdin is read, subsequent calls to util.ReadConfiguration are empty for stdin skaffold.yaml files

These changes are required for supporting stdin for skaffold.yaml's with the line # support that #6955 provides

[codecov]

Codecov Report

Merging #6970 (c2454e9) into main (290280e) will decrease coverage by 1.70%.
The diff coverage is 56.70%.

@@            Coverage Diff             @@
##             main    #6970      +/-   ##
==========================================
- Coverage   70.48%   68.77%   -1.71%     
==========================================
  Files         515      551      +36     
  Lines       23150    25368    +2218     
==========================================
+ Hits        16317    17447    +1130     
- Misses       5776     6738     +962     
- Partials     1057     1183     +126     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/deploy.go	52.00% <ø> (-1.85%)	⬇️
cmd/skaffold/app/cmd/dev.go	84.61% <0.00%> (ø)
cmd/skaffold/app/cmd/render.go	36.66% <0.00%> (-4.72%)	⬇️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/fix.go	68.85% <40.00%> (-7.62%)	⬇️
cmd/skaffold/app/cmd/lint.go	42.85% <42.85%> (ø)
cmd/skaffold/app/cmd/find_configs.go	48.88% <50.00%> (+0.24%)	⬆️
cmd/skaffold/app/skaffold.go	76.19% <70.00%> (-8.43%)	⬇️
... and 177 more

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[MarlonGamez]

lgtm