Fix opam warning

Replace `opam config var` with `opam var` as recommended endlessly by OPAM.

Makefile

@@ -113,14 +113,14 @@ export SAIL_DIR
 EXPLICIT_COQ_SAIL=yes
 else
 # Use sail from opam package
-SAIL_DIR:=$(shell opam config var sail:share)
+SAIL_DIR:=$(shell opam var sail:share)
 SAIL:=sail
 endif
 SAIL_LIB_DIR:=$(SAIL_DIR)/lib
 export SAIL_LIB_DIR
 SAIL_SRC_DIR:=$(SAIL_DIR)/src
 
-LEM_DIR?=$(shell opam config var lem:share)
+LEM_DIR?=$(shell opam var lem:share)
 export LEM_DIR
 
 C_WARNINGS ?=
@@ -353,15 +353,15 @@ riscv_hol_build: generated_definitions/hol4/$(ARCH)/riscvTheory.uo
 ifdef BBV_DIR
   EXPLICIT_COQ_BBV = yes
 else
-  EXPLICIT_COQ_BBV = $(shell if opam config var coq-bbv:share >/dev/null 2>/dev/null; then echo no; else echo yes; fi)
+  EXPLICIT_COQ_BBV = $(shell if opam var coq-bbv:share >/dev/null 2>/dev/null; then echo no; else echo yes; fi)
   ifeq ($(EXPLICIT_COQ_BBV),yes)
     #Coq BBV library hopefully checked out in directory above us
     BBV_DIR = ../bbv
   endif
 endif
 
 ifndef EXPLICIT_COQ_SAIL
-  EXPLICIT_COQ_SAIL = $(shell if opam config var coq-sail:share >/dev/null 2>/dev/null; then echo no; else echo yes; fi)
+  EXPLICIT_COQ_SAIL = $(shell if opam var coq-sail:share >/dev/null 2>/dev/null; then echo no; else echo yes; fi)
 endif
 
 COQ_LIBS = -R generated_definitions/coq Riscv -R generated_definitions/coq/$(ARCH) $(ARCH) -R handwritten_support Riscv_common

prover_snapshots/coq/lib/sail/Makefile

@@ -1,4 +1,4 @@
-HAVE_OPAM_BBV=$(shell if opam config var coq-bbv:share >/dev/null 2>/dev/null; then echo yes; else echo no; fi)
+HAVE_OPAM_BBV=$(shell if opam var coq-bbv:share >/dev/null 2>/dev/null; then echo yes; else echo no; fi)
 
 ifeq ($(HAVE_OPAM_BBV),no)
 BBV_DIR?=../../../bbv/src/bbv

prover_snapshots/hol4/lib/sail/Makefile

@@ -1,4 +1,4 @@
-LEM_DIR?=$(shell opam config var lem:share)
+LEM_DIR?=$(shell opam var lem:share)
 
 LEMSRC =  \
   ../../src/gen_lib/sail2_instr_kinds.lem \