Installing Conda into a global non-writeable depot

We run into this issue at our local supercomputer where we would like to provide a pre-seeded depot
that contains common pkg operations to reduce duplication.

Given a setup like:

```
mkdir /tmp/depot1
mkdir /tmp/depot2
mkdir /tmp/global

sudo mount -o bind,ro /tmp/depot2 /tmp/global

export JULIA_DEPOT_PATH="/tmp/depot2"
julia --project=. -e 'using Pkg; Pkg.add("Conda")'
```

Firstly `]build Conda` fails since we are trying to touch a file in to non-writeable depot
```
export JULIA_DEPOT_PATH="/tmp/depot1:/tmp/global"
julia --project=. -e 'using Pkg; Pkg.build("Conda")'
```

Secondly adding a Conda package also fails since during the build we cached the path
to the depot we got installed into.

```
export JULIA_DEPOT_PATH="/tmp/depot1:/tmp/global"
julia --project=. -e 'using Conda;Conda.add("Numpy")'
```

Any ideas for how we can make the user experience better here?
@lmilechin