Run CI on a stock image, installing Trunk by hand

Instead of building a custom CI image with Trunk pre-installed, let's
try running CI on a stock image, installing Trunk from a binary on every
workflow run.

.forgejo/setup-trunk/action.yaml

@@ -0,0 +1,14 @@
+# set up the Trunk web build system
+#
+#   https://trunkrs.dev
+#
+# the `curl` call is based on David Tolnay's `rust-toolchain` action
+#
+#   https://github.com/dtolnay/rust-toolchain
+#
+runs:
+  using: "composite"
+  steps:
+    - run: rustup target add wasm32-unknown-unknown
+    - run: curl --output - --proto '=https' --tlsv1.2 --retry 10 --retry-connrefused --location --silent --show-error --fail 'https://github.com/trunk-rs/trunk/releases/download/v0.21.12/trunk-x86_64-unknown-linux-gnu.tar.gz' | tar --gunzip --extract --file -
+      working-directory: /usr/local/cargo/bin