Fix EnC CDI cross-validation test process launch on Desktop and bare CI images

The Roslyn cross-validation test shells out to dotnet build to produce a
real Roslyn PDB. The process launch had two problems. It computed the
host path by hand from __SOURCE_DIRECTORY__, which misses on CI images
that carry no repo-local .dotnet at that depth, failing with
Win32Exception before the build starts. It also left UseShellExecute at
its default, which is true on net472 and rejects redirected streams, so
every Desktop test leg failed deterministically with
InvalidOperationException.

Resolve the host like the rest of the test framework via
TestFramework.initialConfig.DotNetExe, which prefers the repo-local
.dotnet and falls back to PATH, and set UseShellExecute to false
explicitly.

Verified: FSharp.Compiler.ComponentTests builds clean;
EncMethodDebugInformationTests 17 passed, 0 failed (net10.0); fantomas
clean on the touched file.

Author: NatElkins

tests/FSharp.Compiler.ComponentTests/CompilerService/EncMethodDebugInformationTests.fs

@@ -300,10 +300,15 @@ let private buildCSharpScratchPdb () =
     )
 
     let psi = System.Diagnostics.ProcessStartInfo()
-    psi.FileName <- Path.Combine(__SOURCE_DIRECTORY__, "..", "..", "..", ".dotnet", "dotnet")
+    // Resolve the dotnet host like the rest of the test framework: repo-local .dotnet
+    // first, PATH fallback otherwise (the hand-rolled path misses on some CI images).
+    psi.FileName <- TestFramework.initialConfig.DotNetExe
     // ProcessStartInfo.ArgumentList does not exist on net472, so build the quoted argument
     // string by hand (projPath is the only argument that can contain spaces).
     psi.Arguments <- $"build \"{projPath}\" -c Debug -p:DebugType=portable -v m"
+    // net472 defaults UseShellExecute to true, which is incompatible with stream
+    // redirection; set it explicitly so the Desktop test legs can start the process.
+    psi.UseShellExecute <- false
     psi.RedirectStandardOutput <- true
     psi.RedirectStandardError <- true
     psi.WorkingDirectory <- workDir