Enable .netrc support for curl

### New feature, improvement proposal

curl supports optionally reading username and password information from a .netrc file if enabled by the appropriate flag (`--netrc-optional`, available since curl 7.9.8 released in 2002). This also matches the default behavior of wget. This flag should be added to the curl invocations without `MVNW_USERNAME` set to allow for this alternative way of setting the credentials if wget is not available on the system.