Fix Dockerfile conda install error for some shells (#92702)
The issue was first solved in [/pull/91371] for CI/CD, but the main Dockerfile in the repo root still has this issue for people trying to test build custom image manually.
Without it the build fails at installing miniconda
```
#14 3.802 Preparing transaction: ...working... done
#14 4.087 Executing transaction: ...working... done
#14 5.713 /root/miniconda.sh: 438: /root/miniconda.sh: [[: not found
#14 5.713
#14 5.713 Installing * environment...
#14 5.713
#14 5.714 /root/miniconda.sh: 444: /root/miniconda.sh: [[: not found
#14 6.050
#14 6.050 CondaFileIOError: '/opt/conda/pkgs/envs/*/env.txt'. [Errno 2] No such
file or directory: '/opt/conda/pkgs/envs/*/env.txt'
#14 6.050
```
With the modification, locally tested build successfully with `make -f ./docker.Makefile` as instructed in the README
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92702
Approved by: https://github.com/seemethere, https://github.com/malfet
