treewide: use ${stdenv.shell} instead of /bin/sh where possible

pkgs/tools/graphics/briss/default.nix

@@ -18,7 +18,7 @@ in stdenv.mkDerivation {
     mkdir -p "$out/bin";
     mkdir -p "$out/share";
     install -D -m444 -t "$out/share" *.jar
-    echo "#!/bin/sh" > "$out/bin/briss"
+    echo "#!${stdenv.shell}" > "$out/bin/briss"
     echo "${jre}/bin/java -Xms128m -Xmx1024m -cp \"$out/share/\" -jar \"$out/share/briss-${version}.jar\"" >> "$out/bin/briss"
     chmod +x "$out/bin/briss"
   '';

pkgs/tools/graphics/povray/default.nix

@@ -34,7 +34,7 @@ stdenv.mkDerivation rec {
   preInstall = ''
     mkdir "$TMP/bin"
     for i in chown chgrp; do
-      echo '#!/bin/sh' >> "$TMP/bin/$i"
+      echo '#!${stdenv.shell}' >> "$TMP/bin/$i"
       chmod +x "$TMP/bin/$i"
       PATH="$TMP/bin:$PATH"
     done