Add a script to test under Emacs 27.2 via nix-emacs-ci

CI does not cover Emacs 27, but Package-Requires now declares it as the
floor, so there has to be a way to check it by hand.

nix-emacs-ci does not expose Emacs 27 on aarch64-darwin (it fails to build
old Emacs on that platform), only on x86_64-darwin, so on Apple Silicon
this goes through Rosetta -- which needs `extra-platforms = x86_64-darwin`
in nix.conf.  The prebuilt binaries are also not cached for x86_64-darwin,
so the first run builds Emacs from source; the nix-emacs-ci revision is
pinned so that later runs reuse it instead of rebuilding.

Author: zonuexe

.github/run-emacs-27-2.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Build this package with Emacs 27.2 from nix-emacs-ci.
+#
+# CI does not cover Emacs 27 (see .github/workflows/test.yml), so this is the
+# way to check the floor declared in Package-Requires by hand.
+#
+# nix-emacs-ci does not expose Emacs 27 on aarch64-darwin, only on
+# x86_64-darwin, so on Apple Silicon this runs through Rosetta.  That needs
+# `extra-platforms = x86_64-darwin` in /etc/nix/nix.conf.  Reaching the
+# prebuilt binaries also needs your user in `trusted-users` there, otherwise
+# the daemon silently ignores the substituter below and builds from source.
+set -euo pipefail
+
+# Pinned rather than master, so that the Emacs you get today is the Emacs you
+# got last time.  nix-emacs-ci pins nixpkgs in its own flake.lock, so following
+# master would change the derivation -- and on a platform without prebuilt
+# binaries (see below) that means rebuilding Emacs from source.
+NIX_EMACS_CI_REV=${NIX_EMACS_CI_REV:-ca88f42462347e8e17859645aa59b156617e98fb}
+NIX_EMACS_CI=${NIX_EMACS_CI:-https://github.com/purcell/nix-emacs-ci/archive/${NIX_EMACS_CI_REV}.tar.gz}
+EMACS_ATTR=${EMACS_ATTR:-emacs-27-2}
+
+case "$(uname -s)/$(uname -m)" in
+  Darwin/arm64) SYSTEM=x86_64-darwin ;;
+  Darwin/*)     SYSTEM=x86_64-darwin ;;
+  Linux/x86_64) SYSTEM=x86_64-linux ;;
+  Linux/aarch64) SYSTEM=aarch64-linux ;;
+  *) echo "unsupported host: $(uname -s)/$(uname -m)" >&2; exit 1 ;;
+esac
+
+cd "$(dirname "$0")/.."
+
+echo "==> Fetching ${EMACS_ATTR} for ${SYSTEM}"
+emacs_path=$(nix-build --no-out-link \
+  --option system "${SYSTEM}" \
+  --option extra-substituters https://emacs-ci.cachix.org \
+  --option extra-trusted-public-keys emacs-ci.cachix.org-1:B5FVOrxhXXrOL0S+tQ7USrhjMT5iOPH+QN9q0NItom4= \
+  -A "${EMACS_ATTR}" \
+  -E "import (fetchTarball \"${NIX_EMACS_CI}\")")
+
+export PATH="${emacs_path}/bin:${PATH}"
+export EMACS="${emacs_path}/bin/emacs"
+
+echo "==> $("${EMACS}" --version | head -1)"
+
+# Stale .elc files silently win over newer .el, which would hide a failure.
+find . -maxdepth 1 -name '*.elc' -delete
+rm -rf .eask
+
+echo "==> eask install-deps"
+eask install-deps
+
+echo "==> eask compile"
+eask compile
+
+echo "==> OK"