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.
