remove esy $HOME hack

Instead of patching the generated `Makefile.coq`, we can simply set
`$HOME` in `Makefile.coq.local`.

Makefile.coq.local

 # Prosa: include pretty documentation targets
 include scripts/coqdocjs/Makefile.coqdocjs
 include scripts/Makefile.alectryon
+
+# Hack to enable esy support:
+#
+# In the esy container, $HOME is hidden, but this causes Coq to complain.
+# We thus include a dummy $HOME value.
+HOME ?= $(DESTDIR)
+export HOME

scripts/Makefile.coq.patch

 --- Makefile.coq.orig	2021-10-04 11:15:22.592822933 +0200
 +++ Makefile.coq	2021-10-04 11:17:18.684584261 +0200
-@@ -15,6 +15,11 @@
- SELF := $(lastword $(MAKEFILE_LIST))
- PARENT := $(firstword $(MAKEFILE_LIST))
- 
-+# In the esy container, $HOME is hidden, but this causes Coq to complain.
-+# We thus include a dummy $HOME value.
-+HOME ?= $(DESTDIR)
-+export HOME
-+
- # This file is generated by coq_makefile and contains many variable
- # definitions, like the list of .v files or the path to Coq
- include Makefile.conf
 @@ -480,6 +485,9 @@
  	$(HIDE)mkdir -p html
  	$(HIDE)$(COQDOC) \