report: build only required figures

Introduces a intermediate derivation that can be imported into the
report derivation, which contains a string cmd that expands the fig
variable as needed.

garlic/sedReport.nix

@@ -0,0 +1,19 @@
+{
+  stdenv
+, fig
+}:
+  stdenv.mkDerivation {
+    name = "report";
+    src = ./.;
+    buildPhase = ''
+      grep -o '@[^ @]*@' report.tex | sed 's/@//g' | sort -u > list
+
+      echo "fig:" > fun.nix
+      echo "'''" >> fun.nix
+      sed 's:\(^.*\)$:sed -i "s;@\1@;''${\1};g" report.tex:g' list >> fun.nix
+      echo "'''" >> fun.nix
+    '';
+    installPhase = ''
+      cp fun.nix $out
+    '';
+  }