ci: major overhaul

- Simplify some workflow steps and auxiliary scripts
- Build library using cmake when not cross-compiling
- Try to fetch fixtures from cache first
- Use `actions-rust-lang/setup-rust-toolchain`

.github/scripts/cross.sh

@@ -1,17 +1,3 @@
-#!/bin/bash
+#!/bin/bash -eu
 
-# set -x
-set -e
-
-if [ "$BUILD_CMD" != "cross" ]; then
-    echo "cross.sh - is a helper to assist only in cross compiling environments" >&2
-    echo "To use this tool set the BUILD_CMD env var to the \"cross\" value" >&2
-    exit 111
-fi
-
-if [ -z "$CROSS_IMAGE" ]; then
-    echo "The CROSS_IMAGE env var should be provided" >&2
-    exit 111
-fi
-
-docker run --rm -v /home/runner:/home/runner -w "$PWD" "$CROSS_IMAGE" "$@"
+exec docker run --rm -v /home/runner:/home/runner -w "$PWD" "$CROSS_IMAGE" "$@"