provisionersdk: extract and embed agent bootstrap scripts (#2886)

This commit extracts the existing hard-coded agent scripts to their own files and embeds them using go:embed.
We can more easily use e.g. shellcheck to validate our agent scripts.

provisionersdk/scripts/bootstrap_darwin.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env sh
+set -eux pipefail
+BINARY_DIR=$(mktemp -d -t coder.XXXXXX)
+BINARY_NAME=coder
+cd "$BINARY_DIR"
+curl -fsSL --compressed "${ACCESS_URL}bin/coder-darwin-${ARCH}" -o "${BINARY_NAME}"
+chmod +x $BINARY_NAME
+export CODER_AGENT_AUTH="${AUTH_TYPE}"
+export CODER_AGENT_URL="${ACCESS_URL}"
+exec ./$BINARY_NAME agent

provisionersdk/scripts/bootstrap_linux.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env sh
+set -eux pipefail
+BINARY_DIR=$(mktemp -d -t coder.XXXXXX)
+BINARY_NAME=coder
+BINARY_URL=${ACCESS_URL}bin/coder-linux-${ARCH}
+cd "$BINARY_DIR"
+if command -v curl >/dev/null 2>&1; then
+	curl -fsSL --compressed "${BINARY_URL}" -o "${BINARY_NAME}"
+elif command -v wget >/dev/null 2>&1; then
+	wget -q "${BINARY_URL}" -O "${BINARY_NAME}"
+elif command -v busybox >/dev/null 2>&1; then
+	busybox wget -q "${BINARY_URL}" -O "${BINARY_NAME}"
+else
+	echo "error: no download tool found, please install curl, wget or busybox wget"
+	exit 1
+fi
+chmod +x $BINARY_NAME
+export CODER_AGENT_AUTH="${AUTH_TYPE}"
+export CODER_AGENT_URL="${ACCESS_URL}"
+exec ./$BINARY_NAME agent

provisionersdk/scripts/bootstrap_windows.ps1

@@ -0,0 +1,9 @@
+# On Windows, VS Code Remote requires a parent process of the
+# executing shell to be named "sshd", otherwise it fails. See:
+# https://github.com/microsoft/vscode-remote-release/issues/5699
+$ProgressPreference = "SilentlyContinue"
+Invoke-WebRequest -Uri ${ACCESS_URL}bin/coder-windows-${ARCH}.exe -OutFile $env:TEMP\sshd.exe
+Set-MpPreference -DisableRealtimeMonitoring $true -ExclusionPath $env:TEMP\sshd.exe
+$env:CODER_AGENT_AUTH = "${AUTH_TYPE}"
+$env:CODER_AGENT_URL = "${ACCESS_URL}"
+Start-Process -FilePath $env:TEMP\sshd.exe -ArgumentList "agent" -PassThru