cmake module ExternalProject running with extra `make install` command

From what I gather when using the cmake module `ExternalProject` the building and installation is taken care of without any outside input. When using `rez-build --install` we can utilize the `REZ_BUILD_INSTALL` variable to query for in the `CMakeLists` file to help create commands.

When executing `rez-build --install` it seems that rez wants to perform a `make install` as well causing a error output:

```bash
make[3]: *** No rule to make target `install'. Stop.
make[2]: *** [boost/src/boost-stamp/boost-install] Error 2
make[1]: *** [CMakeFiles/boost.dir/all] Error 2
make: *** [all] Error 2
10:15:06 ERROR BuildError: The cmake build system failed.
```

I guess it makes sense that we would want to have `make install` if we were performing something else outside of the ExteralProject but when we are just using that operation everything is already taken care of as I can comment out the code block performing the `make install` command from the `CMakeBuildSystem.build` function in cmake.py without causing any issues.

```python
if not retcode and install and "install" not in cmd:
cmd.append("install")

# execute make install within the build env
_pr("\nExecuting: %s" % ' '.join(cmd))
retcode, _, _ = context.execute_shell(command=cmd,
block=True,
cwd=build_path,
actions_callback=callback)
```

Should there be some way to specify that ExternalProject should handle the execution or have I missed something that already specifies that?

Start in cmake.py at CMakeBuildSystem.build and inspect how the install argument, REZ_BUILD_INSTALL, and the ExternalProject module interact. Reproduce the failure with rez-build --install and review the issue discussion for the intended opt-in behavior. Done means ExternalProject builds and installs without an erroneous extra make install, while ordinary CMake projects retain their installation behavior.

Written by the indexing model from the issue text.