Enable cli crossgen step

[dagood]

#418 adds this to unblock the CLI build:

source-build/repos/cli.proj

Lines 65 to 71 in a184e0b

	<!--
	Disable crossgen until we can figure out:
	CROSSGEN : error : Could not load file or assembly 'Microsoft.CodeAnalysis.CSharp, Version=2.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. The system cannot find the file specified.
	CROSSGEN : error : compilation failed for "/work/tarball/src/cli/bin/2/linux-x64/dotnet/sdk/2.1.300-preview2-000001/Microsoft.DotNet.ProjectJsonMigration.dll" (0x80131621)
	-->
	<EnvironmentVariables Include="DISABLE_CROSSGEN=true" />

I'm wondering if this is important to fix in 2.1-preview2 to improve our 2.1 RTM build performance. (I'm not familiar with the kinds of speedup we see, or if there's any direct requirement for crossgenning.)

/cc @dleeapho @dseefeld @peterhuene