paket config add-credentials doesn't work on Linux if the username contains "\"

Description

I can't seem to figure out the way to make paket write the username including the domain using config add-credentials on Linux (in a Docker container, if that matters).

Tech details:

root@15d1470702a7:~/# mono "./.paket/paket.exe" --version
Paket version 5.173.4

root@15d1470702a7:~# mono --version
Mono JIT compiler version 5.12.0.226 (tarball Thu May  3 10:03:41 UTC 2018)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
        TLS:           __thread
        SIGSEGV:       altstack
        Notifications: epoll
        Architecture:  amd64
        Disabled:      none
        Misc:          softdebug
        Interpreter:   yes
        LLVM:          supported, not enabled.
        GC:            sgen (concurrent by default)

The full command:

mono "./.paket/paket.exe" config add-credentials https://my.awesome.feed/nuget/Feed --username domain\user --password pass123 --authtype ntlm

Repro steps

How I tested it:

rm -f /home/jenkins/.config/Paket/paket.config && mono "./.paket/paket.exe" config add-credentials https://my.awesome.feed/nuget/Feed --username domain\user --password pass123 --authtype ntlm && cat /home/jenkins/.config/Paket/paket.config

How else did I try:
domain\user
'domain\user'
"domain\user"
domain\\user
'domain\\user'
"domain\\user"

All the commands resulted in the following paket.config:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <credentials>
    <credential source="https://my.awesome.feed/nuget/Feed" username="domainuser" password="Q+VLnr6M63Pj7a4f6+PHBQ4tKFnkadxCvSH9vtK4EzQfmdCBkY/6oODOdpRO4ChdsUBUJzQ6LzKjXVFqp
bzWudhtHjac+kUKlZWxYlBxnxqvyWznAvdsRAsf76mwFSZYe03uTmdXCRXb5KKCS7UssMmF2w9wAkYo3/wde0YMH5xpEIuoea+53H0fecZi8qBjPrXR4/RH7OHjlvUZfxAZ+c8s7WmDy4Xi+1fwabliT2jwf/xKHd38WFp5Y77K+u
n+8LBvqU9Kb+C9bLyJQ9YIcg==" authType="ntlm" salt="SZrBFTNbUbs=" />
  </credentials>
</configuration>

except for when the username was written as 'domain\\user', in this case the resulting user name was username="domain\\user" (notice two slashes):

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <credentials>
    <credential source="https://my.awesome.feed/nuget/Feed" username="domain\\user" password="c4WOAWKQaVpflX1K3PE0qYWADLvdv6gnWdnY7+pXTlzaPeb6e+wa7bCi48COlW6VUOBW74Ogm9tM1F7
1fXQt/tPYGyWx8wQezSBQKTdnoMZgQPKm5A0Dofa7rCV7Q9Qku6eykP9sj3xErDsSJSYe4C4QthYQ0mRKFPm4G0QZp37woqNcjSqYMqHfoDhiMm1AryMs76A4G9FRD97aKhly4SHQGtS2faTBLOll8M7yYzJRWCUQBYQ9+4XvEUDB
T5TrIZPGjWfya0+3NkhkvKQuFQ==" authType="ntlm" salt="gMkcjC6CU5U=" />
  </credentials>
</configuration>

Expected behavior

I expect there to be a command which would allow me to specify the domain in the user name.

Actual behavior

See Repro steps

Known workarounds

Can't find any

Start with the config add-credentials command and reproduce it under Linux/Mono using the supplied username variants, then inspect /home/jenkins/.config/Paket/paket.config. Done means a domain-qualified username is preserved correctly in the generated configuration and the behavior is covered by a regression test.