Update build-and-test job

Use mamba and don't duplicate builds on local PRs.

.github/workflows/build-test-ci.yml

@@ -4,6 +4,13 @@ on: [push, pull_request]
 
 jobs:
   build-and-test:
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run
+    # by the push to the branch. Without this if check, checks are duplicated since
+    # internal PRs match both the push and pull_request events.
+    if:
+      github.event_name == 'push' || github.event.pull_request.head.repo.full_name !=
+      github.repository
+
     runs-on: ${{ matrix.os }}
 
     defaults:
@@ -26,18 +33,21 @@ jobs:
           channel-priority: true
 
       - name: Show conda installation info
-        run: conda info
-
-      - name: Install dependencies
         run: |
-          conda install --file=requirements.txt -c conda-forge
+          conda info
           conda list
 
+      - name: Install requirements
+        run: |
+          conda install mamba
+          mamba install --file=requirements.txt
+          mamba list
+
       - name: Build and install package
-        run: pip install .
+        run: pip install -e .
 
       - name: Install testing dependencies
-        run: conda install --file=requirements-testing.txt -c conda-forge
+        run: mamba install --file=requirements-testing.txt
 
       - name: Test
         run: |