Move most shell out of Nix string literals

The amount of shell in non-shell files was making me uncomfortable,
and two of these previously contained awkward `''$` escapes.

Apart from forcing one more step to run locally, this is just moving
code around.

build-helpers/build-doom-profile.sh

@@ -0,0 +1,54 @@
+# -*- mode: sh; sh-shell: bash -*-
+
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+mkdir $out $out/loader $out/doomdir $out/profile $out/straight
+ln -s $doomDir/* $out/doomdir/
+# yasnippet logs an error at startup if snippets/ does not exist.
+if ! [[ -e $out/doomdir/snippets ]]; then
+    mkdir $out/doomdir/snippets
+fi
+rm $out/doomdir/init.el
+if [[ -z "$profileName" ]]; then
+    maybeSetProfileDir="(setq doom-profile-dir \"$out/profile\")"
+else
+    maybeSetProfileDir=""
+fi
+substitute $initEl $out/doomdir/init.el \
+    --subst-var maybeSetProfileDir \
+    --subst-var profileName \
+    --subst-var-by userInit "$doomDir/init.el" \
+    --subst-var-by straightBaseDir $out
+ln -sf $doomIntermediates/packages.el $out/doomdir/
+export DOOMDIR=$out/doomdir
+
+# DOOMLOCALDIR must be writable, Doom creates some subdirectories.
+export DOOMLOCALDIR=$(mktemp -d)
+if [[ -n "$profileName" ]]; then
+    export DOOMPROFILELOADFILE=$out/loader/init.el
+    $runtimeShell $doomSource/bin/doomscript $buildProfileLoader \
+        ${noProfileHack:+-u} -n "$profileName" -b "$out"
+
+    # With DOOMPROFILE set, doom-state-dir and friends are HOME-relative.
+    export HOME=$(mktemp -d)
+    export DOOMPROFILE="$profileName";
+fi
+$runtimeShell $doomSource/bin/doomscript $buildProfile
+
+# Similar to audit-tmpdir.sh in nixpkgs.
+if grep -q -F "$TMPDIR/" -r $out; then
+    echo "Doom profile contains a forbidden reference to $TMPDIR/"
+    exit 1
+fi