[protobuf] Only find protoc when cross compiling

[anton-danielsson]

Only use find_program to search for protoc if cross compiling.
This is to make sure a system installed protoc is never used in
a normal situation while still allowing a build profile (or system)
protoc to be used when cross compiling.

This fixes an issue introduced in #4556 that would cause a system installed protoc to have a higher priority when a project is built using cmake-conan (https://github.com/conan-io/cmake-conan).

I created a minimal project and a few scripts to try to make sure it works in the situation I could imagine, see:
https://github.com/anton-danielsson/test-conan-protobuf

Specify library name and version: protobuf

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[conan-center-bot]

All green in build 1 (f1da846119bdb9f01d00ad398163c8f97f888ddf):

• protobuf/3.9.1@:
  All packages built successfully! (All logs)

• protobuf/3.11.3@:
  All packages built successfully! (All logs)

• protobuf/3.11.4@:
  All packages built successfully! (All logs)

• protobuf/3.13.0@:
  All packages built successfully! (All logs)

• protobuf/3.15.5@:
  All packages built successfully! (All logs)

• protobuf/3.12.4@:
  All packages built successfully! (All logs)