Use PIP_CONSTRAINT to fix 20.04 build

.github/workflows/ci.yaml

@@ -56,6 +56,8 @@ jobs:
   build:
     name: Build charm
     uses: canonical/data-platform-workflows/.github/workflows/build_charm.yaml@v31.0.1
+    with:
+      cache: false
 
   integration-test:
     name: Integration test charm

charmcraft.yaml

@@ -53,6 +53,8 @@ parts:
     after:
       - poetry-deps
     poetry-export-extra-args: ['--only', 'main,charm-libs']
+    build-environment:
+      - PIP_CONSTRAINT: constraints.txt  # Workaround for 20.04
     build-packages:
       - libffi-dev  # Needed to build Python dependencies with Rust from source
       - libssl-dev  # Needed to build Python dependencies with Rust from source
@@ -97,5 +99,6 @@ parts:
     stage:
       - LICENSE
       - charm_version
+      - constraints.txt  # Include in *.charm artifact for easier debugging
       - workload_version
       - templates

constraints.txt

@@ -0,0 +1,5 @@
+setuptools_scm < 8.2.0; python_version <= "3.8"
+# 1.8.4 maturin setup.py uses RustBin with env arg, which requires setuptools-rust >= 1.11.0, which
+# does not support python 3.8
+maturin < 1.8.4; python_version <= "3.8"
+setuptools-rust < 1.11; python_version <= "3.8"