emacsPackagesNg.trivialBuild: cleanup and standardize function

No real function change here, but this updates the trivial and melpa builders to
be formatted more consistently with the rest of the builders, and swaps
`eval "$preBuild"` for the more standard `runHook preBuild`.

pkgs/build-support/emacs/melpa.nix

@@ -61,8 +61,7 @@ import ./generic.nix { inherit lib stdenv emacs texinfo; } ({
     ln -s "$NIX_BUILD_TOP/$sourceRoot" "$NIX_BUILD_TOP/working/$ename"
   '';
 
-  buildPhase =
+  buildPhase = ''
-    ''
     runHook preBuild
 
     cd "$NIX_BUILD_TOP"

pkgs/build-support/emacs/trivial.nix

@@ -7,27 +7,22 @@ with lib;
 args:
 
 import ./generic.nix envargs ({
-  #preConfigure = ''
-  #  export LISPDIR=$out/share/emacs/site-lisp
-  #  export VERSION_SPECIFIC_LISPDIR=$out/share/emacs/site-lisp
-  #'';
-
   buildPhase = ''
-    eval "$preBuild"
+    runHook preBuild
 
     emacs -L . --batch -f batch-byte-compile *.el
 
-    eval "$postBuild"
+    runHook postBuild
   '';
 
   installPhase = ''
-    eval "$preInstall"
+    runHook preInstall
 
     LISPDIR=$out/share/emacs/site-lisp
     install -d $LISPDIR
     install *.el *.elc $LISPDIR
 
-    eval "$postInstall"
+    runHook postInstall
   '';
 }