Add CI for the in-repo Spack package (#1544)

Run only on Linux and Spack/develop

closes #1534

.github/workflows/spack.yml

+name: Spack
+
+on:
+  push:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        python-version: [3.8]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+            path: arbor
+      - name: Build Arbor's Spack package against the develop branch
+        run: arbor/scripts/build_spack_package.sh arbor develop

scripts/build_spack_package.sh

+#!/usr/bin/env bash
+# checks out Spack and Arbor and builds it with the package.py from Arbor's repo
+# Spack can be the latest release or the develop branch
+
+set -Eeuo pipefail
+
+if [[ "$#" -ne 2 ]]; then
+    echo "Builds the in-repo Spack package of Arbor against the latest Spack release or a given Spack branch"
+    echo "usage: build_spack_package.sh arbor_source_directory latest_release|develop"
+	exit 1
+fi
+
+trap cleanup SIGINT SIGTERM ERR EXIT
+
+cleanup() {
+  trap - SIGINT SIGTERM ERR EXIT
+  rm -rf "$TMP_DIR"
+}
+
+TMP_DIR=$(mktemp -d)
+ARBOR_SOURCE=$1
+
+ARBOR_DIR=$TMP_DIR/arbor
+mkdir $ARBOR_DIR
+cp -r $ARBOR_SOURCE/* $ARBOR_DIR
+
+cd "$TMP_DIR"
+
+SPACK_DIR=spack
+SPACK_REPO=https://github.com/spack/spack
+SPACK_CUSTOM_REPO=custom_repo
+
+SPACK_VERSION=$2 # latest_release or develop
+SPACK_BRANCH=develop # only used for develop
+
+case $SPACK_VERSION in
+	"develop")
+		git clone --depth 1 --branch $SPACK_BRANCH $SPACK_REPO $SPACK_DIR
+		;;
+	"latest_release")
+		wget "$(curl -sH "Accept: application/vnd.github.v3+json" https://api.github.com/repos/spack/spack/releases/latest | grep browser_download_url |  cut -d '"' -f 4)"
+		tar xfz spack*.tar.gz
+		ln -s spack*/ $SPACK_DIR
+		;;
+	*)
+		echo "SPACK_VERSION" must be \"latest_release\" or \"develop\"
+		exit 1
+esac
+source $SPACK_DIR/share/spack/setup-env.sh
+spack repo create $SPACK_CUSTOM_REPO
+
+mkdir -p $SPACK_CUSTOM_REPO/packages/arbor
+spack repo add $SPACK_CUSTOM_REPO
+
+cp $ARBOR_DIR/spack/package.py $SPACK_CUSTOM_REPO/packages/arbor
+cd $ARBOR_DIR
+spack dev-build arbor@with-package-from-repo