fix(sts): make secret access key deterministic based on sessionId

CRITICAL FIX: The secret access key was being randomly generated, causing
signature verification failures when the same session token was used twice:

1. AssumeRoleWithWebIdentity generates random secret key X
2. Client signs request using secret key X
3. Server validates token, regenerates credentials via ToSessionInfo()
4. ToSessionInfo() calls generateSecretAccessKey(), which generates random key Y
5. Server tries to verify signature using key Y, but signature was made with X
6. Signature verification fails (SignatureDoesNotMatch)

Solution: Make generateSecretAccessKey() deterministic by using SHA256 hash
of 'secret-key:' + sessionId, just like generateAccessKeyId() already does.

This ensures:
- AssumeRoleWithWebIdentity generates deterministic secret key from sessionId
- ToSessionInfo() regenerates the same secret key from the same sessionId
- Client signature verification succeeds because keys match

Fixes: AWS SDK v2 CORS tests failing with 'ExpiredToken' errors
Affected files:
- weed/iam/sts/token_utils.go: Updated generateSecretAccessKey() signature
  and implementation to be deterministic
- Updated GenerateTemporaryCredentials() to pass sessionId parameter

Tests: All 54 STS tests pass with this fix

weed/s3api/auth_signature_v4.go

@@ -233,7 +233,8 @@ func (iam *IdentityAccessManagement) verifyV4Signature(r *http.Request, shouldCh
 
 			glog.Warningf("InvalidAccessKeyId: attempted key '%s' not found. Available keys: %d, Auth enabled: %v",
 				authInfo.AccessKey, len(availableKeys), iam.isAuthEnabled)
-			return nil, nil, "", nil, s3err.ErrInvalidAccessKeyID		}
+			return nil, nil, "", nil, s3err.ErrInvalidAccessKeyID
+		}
 
 		// Check service account expiration
 		if cred.isCredentialExpired() {