Fix corehost RID generation logic for versionless distros

[Saancreed]

The Linux/Bash build script for corehost incorrectly assumes that VERSION_ID will be set after sourcing /etc/os-release, which is not the case on rolling release distributions like Arch Linux. On such systems, calling init_rid_plat initializes base Runtime Identifier to values similar to arch. which ultimately results in malformed RIDs like arch.-x64 and breaks the build process further down the line.

With this change, if VERSION_ID is unset, then the dot will be omitted and final RID will be more correct arch-x64. On distributions with VERSION_ID set in /etc/os-release, this change doesn't affect the RID computation at all.

Applying this fix allows former core-setup repository to correctly build on Arch Linux. Hopefully if this PR is accepted, it will be backported to .NET Core 3.1 too.

(Issue and the fix discovered here, see that issue for more background info.)

[ericstj]

Looks OK to me, I see we have similar logic here:

runtime/src/installer/managed/Microsoft.DotNet.PlatformAbstractions/RuntimeEnvironment.cs

Lines 52 to 57 in aa4eb88

	if (string.IsNullOrEmpty(OperatingSystemVersion))
	{
	return string.Empty;
	}
	return $".{OperatingSystemVersion}";

and here:

runtime/src/installer/corehost/common/pal.unix.cpp

Lines 615 to 619 in aa4eb88

	if (fFoundVersion)
	{
	ridOS.append(_X("."));
	ridOS.append(valVersionID);
	}