treewide: use runtimeShell instead of stdenv.shell whenever possible

Whenever we create scripts that are installed to $out, we must use runtimeShell
in order to get the shell that can be executed on the machine we create the
package for. This is relevant for cross-compiling. The only use case for
stdenv.shell are scripts that are executed as part of the build system.
Usages in checkPhase are borderline however to decrease the likelyhood
of people copying the wrong examples, I decided to use runtimeShell as well.

pkgs/build-support/docker/default.nix

@@ -23,7 +23,8 @@
   writeScript,
   writeText,
   closureInfo,
-  substituteAll
+  substituteAll,
+  runtimeShell
 }:
 
 # WARNING: this API is unstable and may be subject to backwards-incompatible changes in the future.
@@ -119,7 +120,7 @@ rec {
     export PATH=${shadow}/bin:$PATH
     mkdir -p /etc/pam.d
     if [[ ! -f /etc/passwd ]]; then
-      echo "root:x:0:0::/root:${stdenv.shell}" > /etc/passwd
+      echo "root:x:0:0::/root:${runtimeShell}" > /etc/passwd
       echo "root:!x:::::::" > /etc/shadow
     fi
     if [[ ! -f /etc/group ]]; then
@@ -261,7 +262,7 @@ rec {
   # things like `ls` or `echo` will be missing.
   shellScript = name: text:
     writeScript name ''
-      #!${stdenv.shell}
+      #!${runtimeShell}
       set -e
       export PATH=${coreutils}/bin:/bin
       ${text}