Add posix sh env script

This amounted taking the bash env script and removing its use of the
`local` keyword and changing is extension to ".sh". The posix env script
also doesn't source the bash completion script as it uses some
bash-specific features.

This allows the dune binary distro to be used on machines with minimal
shells such as the ash shell in the default environment on alpine linux.

Also adds a test that runs shellcheck on env.sh to make sure it stays
posix-compatible.

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

Author: gridbugs

.github/workflows/test.yml

@@ -0,0 +1,17 @@
+name: test
+on: [push, pull_request]
+jobs:
+
+  shellcheck:
+    name: "Run shellcheck on the shell env script"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: "Install shellcheck"
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y shellcheck
+    - name: "Run shellcheck on the shell env script"
+      run: |
+        # exclude warning for sourcing missing external file
+        shellcheck --exclude=SC1091 extra/share/dune/env/env.sh extra/share/dune/env/env.bash

extra/share/dune/env/env.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+__dune_env() {
+  if [ "$#" != "1" ]; then
+    echo "__dune_env expected 1 argument, got $#"
+    return
+  fi
+  __dune_root="$1"
+
+  # Add dune to PATH unless it's already present.
+  # Affix colons on either side of $PATH to simplify matching (based on
+  # rustup's env script).
+  case :"$PATH": in
+    *:"$__dune_root/bin":*)
+      # Do nothing since the bin directory is already in PATH.
+      ;;
+    *)
+      # Prepending path in case a system-installed dune needs to be overridden
+      export PATH="$__dune_root/bin:$PATH"
+      ;;
+  esac
+}