ci.sh: print used image versions and set defaults

Set default versions for two used images:
```
nilfoundation/zkllvm-template
nilfoundation/proof-market-toolchain
```
These versions match ones in ./github/main.yml and should be updated
together.

With defaults, the script can be used without CI environment and
without setting env variables explicitly.

Resolves #34

scripts/ci.sh

@@ -5,6 +5,14 @@ set -euxo pipefail
 SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
 REPO_ROOT="$SCRIPT_DIR/.."
 
+# Set image versions in the environment before running this script:
+# export ZKLLVM_VERSION=0.0.58
+# export TOOLCHAIN_VERSION=0.0.31
+
+# If unset, default values will be used:
+echo "using nilfoundation/zkllvm-template:${ZKLLVM_VERSION:=0.0.58}"
+echo "using nilfoundation/proof-market-toolchain:${TOOLCHAIN_VERSION:=0.0.31}"
+
 # podman is a safer option for using on CI machines
 if ! command -v podman; then
     DOCKER="docker"