[build] `make leeroy-all` uses `$(_SLN_BUILD)`

Commit 8942eca0 [broke the build][xa554]:

[xa554]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/554/

	error XA0000: Could not determine $(TargetFrameworkVersion) for API level '26.'

The cause of the failure is that we're using `xbuild` within the
`make leeroy-all` target, but [we need to use `xabuild`][xbuild-paths]:

[xbuild-paths]: https://github.com/xamarin/xamarin-android/pull/599#issuecomment-319543023

	Task "GetReferenceAssemblyPaths"
	  Using task GetReferenceAssemblyPaths from Microsoft.Build.Tasks.GetReferenceAssemblyPaths, Microsoft.Build.Tasks.Core, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
	  Looking for framework 'MonoAndroid,Version=v1.0' in root path '/Library/Frameworks/Mono.framework/External/xbuild-frameworks'
	  Found framework definition list '/Library/Frameworks/Mono.framework/External/xbuild-frameworks/MonoAndroid/v1.0/RedistList/FrameworkList.xml' for framework 'MonoAndroid,Version=v1.0'
	Done executing task "GetReferenceAssemblyPaths"

`xbuild` is trying to use the *system-wide*
`/Library/Frameworks/Mono.framework/External/xbuild-frameworks`
directory, which *cannot* contain the now-required
`AndroidApiInfo.xml` files which are needed for probing.

Fix the `make leeroy-all` target to use `$(_SLN_BUILD)`, just as the
`make all` target does, so that the correct paths are used.

build-tools/scripts/BuildEverything.mk

@@ -100,7 +100,7 @@ leeroy: leeroy-all framework-assemblies opentk-jcw
 
 leeroy-all:
 	$(foreach conf, $(CONFIGURATIONS), \
-		$(MSBUILD) $(MSBUILD_FLAGS) $(SOLUTION) /p:Configuration=$(conf) $(_MSBUILD_ARGS) ; )
+		$(_SLN_BUILD) $(MSBUILD_FLAGS) $(SOLUTION) /p:Configuration=$(conf) $(_MSBUILD_ARGS) ; )
 
 framework-assemblies:
 	PREV_VERSION="v1.0"; \