Manually invoke bash for miniconda (#1277)

Fixes build issues failing with:
```
./Miniconda3-latest-Linux-x86_64.sh: 438: ./Miniconda3-latest-Linux-x86_64.sh: [[: not found
```
as seen in e.g.: #1271

Author: ptrblck

common/install_conda.sh

@@ -5,7 +5,8 @@ set -ex
 # Anaconda
 wget -q https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh
 chmod +x  Miniconda3-latest-Linux-x86_64.sh
-./Miniconda3-latest-Linux-x86_64.sh -b -p /opt/conda
+# NB: Manually invoke bash per https://github.com/conda/conda/issues/10431
+bash ./Miniconda3-latest-Linux-x86_64.sh -b -p /opt/conda
 rm Miniconda3-latest-Linux-x86_64.sh
 export PATH=/opt/conda/bin:$PATH
 conda install -y conda-build anaconda-client git ninja