Powershell Hashtable Keys are case-sensitive on Ubuntu 18.04 #7761

On a clean installation of Ubuntu 18.04 from Azure, creating a hashtable in script results in a hashtable with case-sensitive keys.

The LANG environment variable is set to 'C.UTF-8' and both CultureInfo.CurrentCulture and CultureInfo.CurrentUICulture report en-US-POSIX.

The result is scripts that are not case-strict when using hashtable keys will break.

Steps to reproduce

$h = @{HostName = 'host'}
$h.hostname
$h.HostName

Expected behavior

The value 'host' is returned for both $h.HostName and $h.hostname

Actual behavior

No value is returned for $h.HostName is 'host' while $h.hostname returns $null.

Environment data

> $PSVersionTable
 Name                           Value
----                           -----
PSVersion                      6.1.0-rc.1
PSEdition                      Core
GitCommitId                    6.1.0-rc.1
OS                             Linux 4.15.0-1022-azure #23-Ubuntu SMP Thu Aug 16 15:19:42 UTC 2018
Platform                       Unix
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0