Speedup the Continuous Documentation using mamba (GenericMappingTools…

…#813)

* Simplify the vercel build commands

`gmt` is already listed as a dependency in
[environment.yml](https://github.com/GenericMappingTools/pygmt/blob/master/environment.yml),
so running `conda env create -f environment.yml` already has `gmt`
installed. No need to install `gmt` again.

* Use mamba

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

package.json

@@ -1,7 +1,7 @@
 {
   "scripts": {
     "build:miniconda": "curl -o ~/miniconda.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh && bash ~/miniconda.sh -b -p $HOME/miniconda",
-    "build:pygmt": "conda env create -f environment.yml && source activate pygmt && conda install -c conda-forge -y gmt==6.1.1 && make install",
+    "build:pygmt": "conda install mamba -c conda-forge -y && mamba env create -f environment.yml && source activate pygmt && make install",
     "build:docs": "source activate pygmt && cd doc && make all && mv _build/html ../public",
     "build": "export PATH=$HOME/miniconda/bin:$PATH && npm run build:miniconda && npm run build:pygmt && npm run build:docs"
   }