[build] Properly set $(HOME) on Windows (#879)

Commit 6fd330d tried to provide a reasonable default for `$(HOME)`
when building on Windows (...years before we could do that):

> I've arbitrary decided to download the files into
> $(HOME)\android-archives and install them into
> $(HOME)\android-toolchain. On windows, this is
> %HOMEDRIVE%%HOMEPATH%\android-archives and
> %HOMEDRIVE%%HOMEPATH%\android-toolchain.

Where did `%HOMEDRIVE%` and `%HOMEPATH%` come from? They came from an
available Windows machine, and otherwise looked sane and useful.

Unfortunately, [when building in the VSTS build agent][0], neither
`%HOMEDRIVE%` nor `%HOMEPATH%` are set, resulting in `$(HOME)`
not being set at all!

[0]: https://devdiv.visualstudio.com/DevDiv/_build/index?buildId=1004861

Instead of setting `$(HOME)` to `%HOMEDRIVE%%HOMEPATH%` on Windows,
set it to `%USERPROFILE`, which appears to be a more useful
environment variable.

Author: jonathanpeppers

Configuration.props

@@ -38,7 +38,7 @@
     <ManagedRuntime Condition=" '$(ManagedRuntime)' == '' And '$(OS)' != 'Windows_NT' ">mono</ManagedRuntime>
     <ManagedRuntimeArgs Condition=" '$(ManagedRuntimeArgs)' == '' And '$(ManagedRuntime)' == 'mono' ">--debug=casts</ManagedRuntimeArgs>
     <MonoSgenBridgeVersion Condition=" '$(MonoSgenBridgeVersion)' == '' ">5</MonoSgenBridgeVersion>
-    <HOME Condition=" '$(HOME)' == '' ">$(HOMEDRIVE)$(HOMEPATH)</HOME>
+    <HOME Condition=" '$(HOME)' == '' ">$(USERPROFILE)</HOME>
     <AndroidPlatformId Condition=" '$(AndroidPlatformId)' == '' ">$(AndroidApiLevel)</AndroidPlatformId>
     <AndroidPreviousFrameworkVersion Condition=" '$(AndroidPreviousFrameworkVersion)' == '' ">v1.0</AndroidPreviousFrameworkVersion>
     <AndroidToolchainCacheDirectory Condition=" '$(AndroidToolchainCacheDirectory)' == '' ">$(HOME)\android-archives</AndroidToolchainCacheDirectory>