Docs: warn about default variables set by conda

[EZoni]

Adding a warning about conda setting default environment variables when installing CMake, as discussed during the developer meeting.

It will be displayed at the onde of the conda section (https://warpx.readthedocs.io/en/latest/install/dependencies.html#conda-linux-macos-windows):

[RemiLehe]

In the documentation: could you add the command line that a user should type in order to clear the variable?

[EZoni]

@RemiLehe Does this look good to you?

Suggested change

	.. warning::
	When you install CMake in a conda environment, conda sets several environment variables, e.g., ``CXXFLAGS``, to default values that are specific to the conda environment. Such values are concatenated to the values set by WarpX during buildsystem generation. In order to avoid unintended side effects (e.g., compiling with ``-O2`` instead of ``-O3``), please consider clearing ``CXXFLAGS`` (and/or other relevant variables) before generating the buildsystem and compiling.
	.. warning::
	When you install CMake in a conda environment, conda sets several environment variables, e.g., ``CXXFLAGS``, to default values that are specific to the conda environment. Such values are concatenated to the values set by WarpX during buildsystem generation. In order to avoid unintended side effects (e.g., compiling with ``-O2`` instead of ``-O3``), please consider clearing ``CXXFLAGS`` and/or other relevant variables (e.g., via ``export CXXFLAGS=""``) before generating the buildsystem and compiling.