sapling: fix nodejs path, add enableMinimal option

This change fixes the path to 'nodejs' in the Sapling scripts, so that
the 'sl web' command works OOTB even if the user has a fresh `$PATH`
without node itself.

However, this is really a developer-only tool, and isn't needed just to
e.g. clone repositories. In particular, a 'fetchSapling' codepath would
not need it; therefore we make it optional, but turned on by default.
The intention is to have a 'saplingMinimal' expression which can be used
for that path.

NOTE: this does NOT add a 'saplingMinimal' expression to
all-packages.nix; that would just result in more Hydra churn, so we
avoid it for now.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

pkgs/applications/version-management/sapling/default.nix

@@ -17,6 +17,8 @@
 , CoreFoundation
 , CoreServices
 , Security
+
+, enableMinimal ? false
 }:
 
 let
@@ -129,10 +131,17 @@ let
     # 1) This applies on all systems (so no conditional a la postFixup)
     # 2) This doesn't require any kind of fixup itself, so we leave it out
     #    of postFixup for that reason, too
+    # 3) If asked, we optionally patch in a hardcoded path to the 'nodejs' package,
+    #    so that 'sl web' always works
+    # 4) 'sl web' will still work if 'nodejs' is in $PATH, just not OOTB
     preFixup = ''
       sitepackages=$out/lib/${python38Packages.python.libPrefix}/site-packages
       chmod +w $sitepackages
       cp -r ${isl} $sitepackages/edenscm-isl
+    '' + lib.optionalString (!enableMinimal) ''
+      chmod +w $sitepackages/edenscm-isl/run-isl
+      substituteInPlace $sitepackages/edenscm-isl/run-isl \
+        --replace 'NODE=node' 'NODE=${nodejs}/bin/node'
     '';
 
     postFixup = lib.optionalString stdenv.isLinux ''