[ci] Use '$(XAShouldAnalyzeAssembly)'=='True' (#6382)

Context: ad5f6e8

The CI nightly pipeline is failing with:

	/Users/builder/azdo/_work/4/s/xamarin-android/build-tools/scripts/RoslynAnalyzers.projitems(3,14):
	error MSB4113: Specified condition " $(XAShouldAnalyzeAssembly) " evaluates to "" instead of a boolean.

The cause of the problem is commit ad5f6e8, which implicitly
requires that `$(XAShouldAnalyzeAssembly)` have a "boolean" value,
i.e. `True` or `False`.  The empty string is not a valid boolean
value.

If `$(XAShouldAnalyzeAssembly)` isn't set -- as can be the case on
the nightly CI run -- then an MSB4113 error results.

Update the `<ItemGroup/>` within
`build-tools/scripts/RoslynAnalyzers.projitems` to use a "safe"
boolean expression:

	<ItemGroup Condition=" '$(XAShouldAnalyzeAssembly)' == 'True' ">

This will work reliably even if `$(XAShouldAnalyzeAssembly)` isn't set.

Author: jpobst

build-tools/scripts/RoslynAnalyzers.projitems

@@ -1,6 +1,6 @@
 <Project>
 
-  <ItemGroup Condition=" $(XAShouldAnalyzeAssembly) ">
+  <ItemGroup Condition=" '$(XAShouldAnalyzeAssembly)' == 'True' ">
     <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="3.3.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>