ci: update chisel (#622)

A new version of chisel-releases breaks compatibility with chisel
binaries older than v0.8.1. Note that we can't use the chisel snap
because the tests write into /tmp and the strict snap can't do that.

Ref: canonical/rockcraft#449

.github/workflows/tests.yaml

@@ -79,9 +79,11 @@ jobs:
           sudo apt install -y ninja-build cmake scons \
                               autoconf automake autopoint gcc git gperf help2man libtool texinfo
           # Build Chisel: needs a newer version of go than is available in Ubuntu 20.04
+          # note: we can't use the snap here because it has strict confinement and can't access
+          # the test files in /tmp/pytest-*
           sudo snap install --classic --channel=latest/stable go
           git clone https://github.com/canonical/chisel.git chisel-tmp
-          cd chisel-tmp && git checkout f0bff5a30dfdcb400b # known "good" commit until Chisel has versions
+          cd chisel-tmp && git checkout v0.8.1
           go mod download
           sudo go build -o /usr/bin ./...
           chisel --help