Install gen-admin-password alongside vetting

proxmox-install.sh + install.sh left operators with no way to
generate the bcrypt hash on the LXC — 'vetting gen-admin-password'
was suggested in the post-install message but the binary has no
subcommands. Cross-build gen-admin-password-linux-amd64 during the
one-liner flow and drop it into /usr/local/bin.

Makefile

@@ -32,6 +32,10 @@ agent-linux: ## Cross-build agent for linux-amd64 (consumed by live-image build)
 gen-admin-password: ## Build the bcrypt password generator
 	go build -o bin/gen-admin-password$(if $(filter Windows%,$(UNAME_S)),.exe,) ./tools/gen-admin-password
 
+.PHONY: gen-admin-password-linux
+gen-admin-password-linux: ## Cross-build the bcrypt password generator for linux-amd64
+	$(GOOS_LINUX) go build -ldflags="$(LDFLAGS)" -o bin/gen-admin-password-linux-amd64 ./tools/gen-admin-password
+
 .PHONY: tidy
 tidy: ## go mod tidy
 	go mod tidy