Update bump-agent-version to use gsed

This means that we can use the same sed invocation on both macos and linux, but macos users will have to `brew install gsed`

Author: moskyb

Makefile

@@ -204,10 +204,15 @@ update-stack: build/aws-stack.yml env-STACK_NAME
 
 AGENT_VERSION ?= $(shell curl -Lfs "https://buildkite.com/agent/releases/latest?platform=linux&arch=amd64" | grep version | cut -d= -f2)
 
+SED ?= sed
+ifeq ($(shell uname), Darwin)
+	SED = gsed
+endif
+
 bump-agent-version:
-	sed -Ei "s/\[Buildkite Agent v.*\]/[Buildkite Agent v$(AGENT_VERSION)]/g" README.md
-	sed -Ei "s/AGENT_VERSION=.+/AGENT_VERSION=$(AGENT_VERSION)/g" packer/linux/scripts/install-buildkite-agent.sh
-	sed -Ei "s/\\\$$AGENT_VERSION = \".+\"/\$$AGENT_VERSION = \"$(AGENT_VERSION)\"/g" packer/windows/scripts/install-buildkite-agent.ps1
+	$(SED) -Ei "s/\[Buildkite Agent v.*\]/[Buildkite Agent v$(AGENT_VERSION)]/g" README.md
+	$(SED) -Ei "s/AGENT_VERSION=.+/AGENT_VERSION=$(AGENT_VERSION)/g" packer/linux/scripts/install-buildkite-agent.sh
+	$(SED) -Ei "s/\\\$$AGENT_VERSION = \".+\"/\$$AGENT_VERSION = \"$(AGENT_VERSION)\"/g" packer/windows/scripts/install-buildkite-agent.ps1
 
 validate: build/aws-stack.yml
 	aws --no-cli-pager cloudformation validate-template \