checkout specific irmin commit

Author: 3Rafal

regression/build-irmin

@@ -1,6 +1,7 @@
 #!/bin/bash
 git clone https://github.com/mirage/irmin.git
 cd irmin
+git checkout 8da4d16e7cc8beddfc8a824feca325426bae08a9
 sudo apt install -y gnuplot-x11 libgmp-dev pkg-config libffi-dev
 opam install . --deps-only --with-test --no-checksums -y
 opam exec -- dune build

regression/dune

@@ -6,5 +6,4 @@
   (= true %{env:REGRESSION=false}))
  (applies_to :whole_subtree)
  (deps
-  %{bin:build-irmin})
-)
+  %{bin:build-irmin}))