scripts: make absolute paths consistent

Change absolute binary paths in scripts to /usr/bin/env or make them
relative.
This allows the scripts to be used on linux distributions
like NixOS, where binaries are not located in /usr/ or /bin/.

Makefile

@@ -1,4 +1,4 @@
-SHELL = /bin/bash
+SHELL = bash
 TAG := $(shell echo `git describe --abbrev=8 --tags`-`git rev-parse --abbrev-ref HEAD` | sed 's/-\([0-9]\)-/-00\1-/; s/-\([0-9][0-9]\)-/-0\1-/; s/-\(HEAD\|master\)$$//')
 LAST_TAG := $(shell git describe --tags --abbrev=0)
 NEW_TAG := $(shell echo $(LAST_TAG) | perl -lpe 's/v//; $$_ += 0.01; $$_ = sprintf("v%.2f", $$_)')