build: fix tls: failed to verify certificate: x509: negative serial number

Before Go 1.23, x509.ParseCertificate accepted certificates with
negative serial numbers. Rejecting these certificates caused a small
number of users to see this error.

From Go 1.23 debug flags can be added to go.mod so this change adds a
debug flag to ensure negative serial numbers are still allowed since
this is a spec violation, not a security issue.

See: https://forum.rclone.org/t/ssl-validation-broken-between-v1-69-1-latest-version/

go.mod

@@ -2,6 +2,10 @@ module github.com/rclone/rclone
 
 go 1.24.4
 
+godebug (
+    x509negativeserial=1
+)
+
 require (
 	bazil.org/fuse v0.0.0-20230120002735-62a210ff1fd5
 	github.com/Azure/azure-sdk-for-go/sdk/azcore v1.20.0