allow custom environment variables to be set when launching a kernel

[brettfo]

This also fixes a longstanding issue where we weren't properly refreshing the kernelLaunchArguments.

An example in the user's settings.json:

"dotnet-interactive.kernelEnvironmentVariables": {
    "THE_ANSWER": "42",
    "PI": "EXACTLY_3"
}

After setting those values, executing a PowerShell cell will report:

$env:THE_ANSWER

result:

42