Since we are now able to trace shared compilation builds on Linux and macOS
(starting from .NET Core 3), and always were able to on Windows, there is
no need to set `UseSharedCompilation=false` in those cases. This may have a
positive performance impact, as shared compilation is generally faster then
non-shared compilation.
This commit fixes a few issues that were identified during the last dist upgrade,
and which were introduced/revealed on 836daaf07b.
- Expand environment variables that are passed from `lgtm.yml` to the autobuilder,
for example `solution: $LGTM_SRC/mysolution.sln`.
- Distinguish between when a build rule is applied automatically and when it is applied
manually via `lgtm.yml`.
- Catch `FileNotFoundException`s when parsing project files and solution files.
When determining the target of `msbuild` or `dotnet build`, first look for `.proj`
files, then `.sln` files, and finally `.csproj`/`.vcxproj` files. In all three cases,
choose the project/solution file closest to the root.
