avoid pipe in dune

[hannesm]

I got since I updated the OS of our reproducible build infrastructure sporadic but way too often an exit 141 pointing to this dune stanza:

546.839s:#=== ERROR while compiling mirage-unikernel-mollymawk-hvt.0.0.1 ===============#
546.839s:# context              2.5.1 | freebsd/x86_64 |  | git://git.robur.coop/robur/unikernel-repo.git
546.839s:# path                 /tmp/orb-build
546.839s:# command              /usr/local/bin/gmake build
546.839s:# exit-code            2
546.839s:# env-file             /tmp/opam-root-43349/mirage-unikernel-mollymawk-hvt-43349-ad6023.env
546.839s:# output-file          /tmp/opam-root-43349/mirage-unikernel-mollymawk-hvt-43349-ad6023.out
546.839s:### output ###
546.839s:# dune build --profile release --root . ./dist
546.839s:# gmake[1]: Entering directory '/tmp/orb-build/_build/solo5/duniverse/mirage-solo5/lib/bindings'
546.839s:# ocamlfind -toolchain solo5 ocamlopt    -c -o clock_stubs.o clock_stubs.c
546.839s:# ocamlfind -toolchain solo5 ocamlopt    -c -o mm_stubs.o mm_stubs.c
546.839s:# ocamlfind -toolchain solo5 ocamlopt    -c -o main.o main.c
546.839s:# ar r libmirage-solo5_bindings.a clock_stubs.o mm_stubs.o main.o
546.839s:# gmake[1]: Leaving directory '/tmp/orb-build/_build/solo5/duniverse/mirage-solo5/lib/bindings'
546.839s:# ar: warning: creating libmirage-solo5_bindings.a
546.839s:# File "duniverse/Zarith/dune", lines 85-92, characters 0-146:
546.839s:# 85 | (rule
546.839s:# 86 |  (deps META)
546.839s:# 87 |  (action
546.839s:# 88 |   (with-stdout-to
546.839s:# 89 |    zarith_version.ml
546.839s:# 90 |    (progn
546.839s:# 91 |     (run echo "let")
546.839s:# 92 |     (bash "grep \"version\" META | head -1")))))
546.839s:# Context: solo5
546.839s:# Command exited with code 141.
546.839s:# File "duniverse/Zarith/dune", lines 85-92, characters 0-146:
546.839s:# 85 | (rule
546.839s:# 86 |  (deps META)
546.839s:# 87 |  (action
546.839s:# 88 |   (with-stdout-to
546.840s:# 89 |    zarith_version.ml
546.840s:# 90 |    (progn
546.840s:# 91 |     (run echo "let")
546.840s:# 92 |     (bash "grep \"version\" META | head -1")))))
546.840s:# Command exited with code 141.

From https://builds.robur.coop/job/mollymawk/build/d3976fa9-ad07-499d-ac8d-2d06e9c5a517/console as example

Now, reading up on 141, I figured the | head -1 may send a SIGPIPE to grep if it exited while grep is still running.... so why not remove the head -1 and use a grep expression that matches exactly one version = in the META file?

Well, please take a look and tell me what you think about it.