Improve Apple Silicon Mac support

Following on from #1237, with new discoveries!

Turns out that in many cases, Apple Silicon Macs will in fact run Bash in amd64 mode, which in turn means the reported architecture is amd64, even though it's an Apple Silicon Mac. Homebrew has worked around this in Homebrew/brew#7995, and we more or less follow their lead here.

This makes it so that we _always_ show our little notice on Apple Silicon Macs, whether their shell is in amd64 or arm64 mode.

install.sh

@@ -31,19 +31,45 @@ else
   PLATFORM="linux"
 fi
 
+# On Apple Silicon Macs, the architecture reported by `uname` depends on the
+# architecture of the shell, which is in turn influenced by the *terminal*,
+# as *child processes prefer their parents' architecture*.
+# 
+# This means that for Terminal.app with the default shell it will be arm64,
+# but x86_64 for people using (pre-3.4.0 builds of) iTerm2 or x86_64 shells.
+#
+# Based on logic in Homebrew at https://github.com/Homebrew/brew/pull/7995
+function buildkite-cpu-arm64 {
+  [[ "$PLATFORM" == "darwin" && "$(/usr/sbin/sysctl -n hw.optional.arm64 2> /dev/null)" == "1" ]]
+}
+
+# If we are running on macOS and on Apple Silicon, we force the ARCH to amd64
+# to take advantage of Rosetta 2, and emit a special message for those users.
+function buildkite-apple-silicon-check {
+  if buildkite-cpu-arm64; then
+    ARCH="amd64"
+    echo -e "\n\033[35mHi there, adventurer! \033[36mWe don't yet have a binary for macOS on Apple Silicon; relying on Rosetta 2 and using $ARCH instead!\033[0m"
+  fi
+}
+
 if [ -n "$BUILDKITE_INSTALL_ARCH" ]; then
   ARCH="$BUILDKITE_INSTALL_ARCH"
   echo "Using explicit arch '$ARCH'"
 else
   case $MACHINE in
     *amd64*)   ARCH="amd64"   ;;
-    *x86_64*)  ARCH="amd64"   ;;
+    *x86_64*)
+      ARCH="amd64"
+      # x86_64 is reported by Apple Silicon Macs
+      # when the shell is running inside Rosetta 2
+      buildkite-apple-silicon-check
+      ;;
     *arm64*)
       ARCH="arm64"
-      if [[ "$PLATFORM" == "darwin" ]]; then
-        ARCH="amd64"
-        echo -e "\n\033[35mHi there, adventurer! \033[36mWe don't yet have a binary for macOS on Apple Silicon; relying on Rosetta 2 and using $ARCH instead!\033[0m"
-      fi
+      # ARM64 is the native arch on Apple Silicon Macs,
+      # but we only have amd64 builds to offer, so this
+      # check can override for compatibility
+      buildkite-apple-silicon-check
       ;;
     *armv8*)   ARCH="arm64"   ;;
     *armv7*)   ARCH="armhf"   ;;