CMake: Fix msh3-config.cmake configuration

[dg0yt]

• Chainloading the exported config has an expression based on ${SELF_DIR}.
  • The actual value is not known at msh3 build time.
  • Due to lack of @ONLY, configure_file substitutes the expression with an empty string at msh3 build time. This makes the exported config unusable.
  • The desired values is available as ${CMAKE_CURRENT_LIST_DIR} when the configuration is used.
  • Updating the expression and using @ONLY fixes chainloading and simplifies maintenance of msh4-config.cmake.in by separating build-time substitutions (@VAR@) from usage-time substitutions (${VAR}). This is common practice in generating CMake config files.

(The cmake config template still carries the @FILENAME_DEP_REPLACE@ placeholder which is probably copied from msquic but not set in msh3.)

[nibanks]

Can you please provide more description as to why this is necessary?

[dg0yt]

Update PR description.