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/tree-sitter.sh

@@ -1,28 +1,9 @@
-#!/bin/bash
-
-# set -x
-set -e
-
-if [ -z "$ROOT" ]; then
-    echo "The ROOT env var should be set to absolute path of a repo root folder" >&2
-    exit 111
-fi
-
-if [ -z "$TARGET" ]; then
-    echo "The TARGET env var should be equal to a \`cargo build --target <TARGET>\` command value" >&2
-    exit 111
-fi
+#!/bin/bash -eu
 
 tree_sitter="$ROOT"/target/"$TARGET"/release/tree-sitter
 
-if [ "$BUILD_CMD" == "cross" ]; then
-    if [ -z "$CROSS_RUNNER" ]; then
-        echo "The CROSS_RUNNER env var should be set to a CARGO_TARGET_*_RUNNER env var value" >&2
-        echo "that is available in a docker image used by the cross tool under the hood" >&2
-        exit 111
-    fi
-
-    cross.sh $CROSS_RUNNER "$tree_sitter" "$@"
+if [[ $BUILD_CMD == cross ]]; then
+  cross.sh "$CROSS_RUNNER" "$tree_sitter" "$@"
 else
-    "$tree_sitter" "$@"
+  exec "$tree_sitter" "$@"
 fi