[html] update 'short installation instructions' for Linux

html/get-frama-c.html

@@ -28,13 +28,17 @@ description: Download and install the Frama-C analysis platform now!
 <span class="shell-linux-comment"># 1. Install <a href="https://opam.ocaml.org/">opam</a> (OCaml package manager)</span>
 sudo apt install opam <span class="shell-linux-comment"># or dnf, pacman, etc.</span>
 
+<span class="shell-linux-comment"># Or you can <a target="_blank" class="shell-link shell-linux" href="https://github.com/ocaml/opam/releases">download an opam binary</a>, put it in the PATH</span>
+<span class="shell-linux-comment"># and run it directly (no sudo required)</span>
 
-<span class="shell-linux-comment"># 2. Install Frama-C's dependencies</span>
-opam install depext
-opam depext frama-c
+<span class="shell-linux-comment"># 2. Initialize opam (install an OCaml compiler)</span>
+opam init --compiler 4.14.1 <span class="shell-linux-comment"># may take a while</span>
+eval $(opam env)
+
+<span class="shell-linux-comment"># 3. Install Frama-C (including dependencies)</span>
+opam install frama-c <span class="shell-linux-comment"># opam will install system package dependencies;</span>
+<span class="shell-linux-comment">                     # may require sudo</span>
 <br>
-<span class="shell-linux-comment"># 3. Install Frama-C itself</span>
-opam install frama-c
 
 <span class="shell-linux-comment"># Issues? Problems? <a target="_blank" class="shell-link shell-linux" href="https://git.frama-c.com/pub/frama-c/blob/master/INSTALL.md">Click here for help/instructions</a></span></pre>
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