Move GitHub Actions CI scripts into separate files

Break the workflow into smaller stages (Configure, Build, Install,
Package, etc.) so that you can see exactly which stage failed in the
GitHub Actions run log.

Create a separate Bash CI script for each job stage (configure.sh,
build.sh, install.sh, package.sh, etc.) to reduce the size of the main
YAML workflow file and enable Bash syntax highlighting.

Close #917

scripts/ci/build.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+((${BASH_VERSION%%.*} >= 4)) || { echo >&2 "$0: Error: Please upgrade Bash."; exit 1; }
+
+set -euxo pipefail
+
+if [[ "${OSTYPE}" == msys* ]]; then # Windows
+
+    cpus="${NUMBER_OF_PROCESSORS}"
+
+elif [[ "${OSTYPE}" == darwin* ]]; then # macOS
+
+    cpus="$(sysctl -n hw.ncpu)"
+
+else # Linux & others
+
+    cpus="$(nproc)"
+
+fi
+
+# Build Audacity
+cmake --build build -j "${cpus}" --config "${AUDACITY_BUILD_TYPE}"