misc: Use /usr/bin/env for a more generic shebang

Use path based SHELL in Makefiles. It was suggested that this is a
better solution for make for cases when there is no /usr/bin/env.

See: https://github.com/purpleidea/mgmt/pull/694#discussion_r1015596204

lang/Makefile

@@ -27,7 +27,7 @@
 # additional permission if he deems it necessary to achieve the goals of this
 # additional permission.
 
-SHELL = /usr/bin/env bash
+SHELL = bash
 .PHONY: all build clean fuzz
 
 all: build

lang/core/embedded/provisioner/main.mcl

@@ -794,7 +794,7 @@ class base:host($name, $config) {
 		# passed into the provisioner. It's just now in a deploy subdir.
 		# If it's a dir, then this becomes the empty strings.
 		# XXX: The deploy could instead happen over the network to etcd.
-		"echo '#!/bin/bash' > ${firstboot_scripts_dir}mgmt-deploy.sh && echo '${handoff_binary_path} deploy lang --seeds=http://127.0.0.1:2379 --no-git --module-path=${deploy_dir_modules} ${deploy_dir}${handoff_code_chunk}' >> ${firstboot_scripts_dir}mgmt-deploy.sh && chmod u+x ${firstboot_scripts_dir}mgmt-deploy.sh"
+		"echo '#!/usr/bin/env bash' > ${firstboot_scripts_dir}mgmt-deploy.sh && echo '${handoff_binary_path} deploy lang --seeds=http://127.0.0.1:2379 --no-git --module-path=${deploy_dir_modules} ${deploy_dir}${handoff_code_chunk}' >> ${firstboot_scripts_dir}mgmt-deploy.sh && chmod u+x ${firstboot_scripts_dir}mgmt-deploy.sh"
 	}
 
 	# TODO: Do we want to signal an http:flag if we're a "default" host?

lang/fuzz/Makefile

@@ -27,7 +27,7 @@
 # additional permission if he deems it necessary to achieve the goals of this
 # additional permission.
 
-SHELL = /usr/bin/env bash
+SHELL = bash
 .PHONY: all fuzz
 
 all: fuzz