Don't set execution target when compiling Q# libraries

[rmshaffer]

When compiling a Q# executable for submission to Azure Quantum, IQ# provides the execution target and runtime capability to the compiler so that target-specific compilation can be performed. However, the execution target and runtime capability should not be provided when compiling Q# snippets or projects as libraries. This PR ensures that these parameters are only provided when compiling a Q# executable.

Keeping this PR as draft until the following items are resolved:

• This PR currently results in a degraded experience when displaying targeting-related errors to the user; the errors show up as an error in the generated entry point, rather than pointing to the actual source code with the error. We need Better targeting-related error messages for multi-project Q# applications qsharp-compiler#736 to be implemented in order to correctly display the underlying error.