Fix Conda CI on Linux avoiding to use the defaults channel

[traversaro]

The CI has been using the defaults channel (even if this was not wanted) for a long time, see conda-incubator/setup-miniconda#186 . However, since yesterday this created a compilation error:

2021-07-30T02:25:20.1711320Z /usr/share/miniconda/envs/test/x86_64-conda-linux-gnu/include/c++/9.3.0/ctime:80:11: error: '::timespec_get' has not been declared
2021-07-30T02:25:20.1712202Z    80 |   using ::timespec_get;
2021-07-30T02:25:20.1712597Z       |           ^~~~~~~~~~~~

For this reason, better to cleanup the use of conda channels in the CI, making sure that we don't use defaults at all.

Similar to robotology/robometry#141 .

[S-Dafarra]

I guess this may cause merge conflicts with devel right?

[traversaro]

I guess this may cause merge conflicts with devel right?

Yes, I will handle that when forward merging.