dapp/seth: fix nix-run invocations again

This time for real.

The `--expr` argument semantics are that the installable is an attribute
in the set the expression evaluates to, so we need to move `$expr`
outside the nix expression. If that makes sense.

Author: asymmetric

src/dapp/libexec/dapp/dapp---nix-run

@@ -16,4 +16,4 @@ have() { command -v "$1" >/dev/null; }
 
 expr="$1"; shift
 
-nix run --impure --expr "with import $DAPPTOOLS {}; $expr" -- "$@"
+nix run --impure --expr "import $DAPPTOOLS {}" "$expr" -- "$@"

src/seth/libexec/seth/seth---nix-run

@@ -16,4 +16,4 @@ have() { command -v "$1" >/dev/null; }
 
 expr="$1"; shift
 
-nix run --impure --expr "with import $DAPPTOOLS {}; $expr" -- "$@"
+nix run --impure --expr "import $DAPPTOOLS {}" "$expr" -- "$@"