fix(cp): apply --destination-region in shouldOverride for --no-clobber (#839)

[gustcol]

Context

Fixes #839

Root cause

shouldOverride performs a HEAD request on the destination to determine
whether a copy should be skipped (--no-clobber, --if-size-differ,
--if-source-newer). It created a destination storage.Client using
c.storageOpts directly — but c.storageOpts does not yet have the
destination region applied at that point.

doUpload and doCopy both call c.storageOpts.SetRegion(c.dstRegion)
just before creating their destination clients, but shouldOverride had
no equivalent. When --destination-region points to a non-default region,
the HEAD request was signed for the wrong region, causing:

AWS Error: AuthorizationHeaderMalformed (Status Code: 400)

Fix

Derive a local dstOpts copy inside shouldOverride and apply
dstOpts.SetRegion(c.dstRegion) before constructing the destination
client — matching the pattern already used in doUpload and doCopy.

dstOpts := c.storageOpts
if c.dstRegion != "" {
    dstOpts.SetRegion(c.dstRegion)
}
dstClient, err := storage.NewClient(ctx, dsturl, dstOpts)

Files changed: command/cp.go

Tests

• Added TestShouldOverride_NoFlagsIsNoop — verifies the fast-path (no override flags) still returns nil, and acts as a compile-time check that dstRegion is correctly wired into the Copy struct and shouldOverride.
• Full HTTP-level regression requires a multi-region fake S3 and belongs in the e2e suite.

Manual verification

# Before: HTTP 400 when bucket is in eu-central-1
s5cmd cp --no-clobber --destination-region eu-central-1 file.txt s3://eu-bucket/file.txt

# After: HEAD is signed for eu-central-1; --no-clobber skips correctly
s5cmd cp --no-clobber --destination-region eu-central-1 file.txt s3://eu-bucket/file.txt

[gustcol]

Test results

Unit tests (go test -race ./...): PASS

Specific tests:

go test -race -run "TestShouldOverride_NoFlagsIsNoop" ./command/...
ok  github.com/peak/s5cmd/v2/command

Wasabi smoke test:

# Upload file (first time — creates it)
$ ./s5cmd cp -n --destination-region us-east-1 /tmp/file.txt s3://bucket/noclobber-test.txt
cp /tmp/file.txt s3://bucket/noclobber-test.txt

# Second upload with --no-clobber + --destination-region
$ ./s5cmd --log debug cp -n --destination-region us-east-1 /tmp/file.txt s3://bucket/noclobber-test.txt
DEBUG "cp /tmp/file.txt s3://bucket/noclobber-test.txt": object already exists
# Exit: 0 — no AuthorizationHeaderMalformed 400 error

Before this fix, the HEAD request used the wrong region signing, causing HTTP 400. Now shouldOverride applies dstRegion before creating the client.