webview build [nfc]: make all variables readonly where possible

tools/build-webview

@@ -62,7 +62,7 @@ export -f z_rsync
 ################################################################################
 
 # Chdir to the current git repo's root.
-_root="$(git rev-parse --show-toplevel)"
+readonly _root="$(git rev-parse --show-toplevel)"
 cd "$_root"
 
 # Ensure that we have access to a functional `readlink -f`.
@@ -98,6 +98,9 @@ fi
 # Make $_dest absolute, if it isn't already.
 _dest=$(readlink -m "$_dest")
 
+# Argument parsing has concluded; argument variables are no longer mutable.
+readonly _target _dest _sanity_checks
+
 # Sanity-check _dest and _target, if not explicitly disabled.
 if (( _sanity_checks )); then
     case "$_target" in
@@ -142,13 +145,13 @@ fi  # if _sanity_checks
 
 # Set _staging. This is our local staging directory; once it's assembled, it'll
 # be synced over to _dest.
-_staging=src/webview/static
+readonly _staging=src/webview/static
 if [ ! -d "$_staging" ]; then
     err "cannot find asset staging directory '$_staging'";
 fi
 
 # The auxiliary build script directory.
-_aux_script_dir="$_root"/tools/build-webview.d
+readonly _aux_script_dir="$_root"/tools/build-webview.d
 
 ################################################################################
 # Pre-sync: additional build steps

tools/build-webview.d/katex.all.pre

@@ -1,12 +1,12 @@
 #!/usr/bin/env bash
 set -eu
 
-_src=${ROOT}/node_modules/katex/dist
-_dest="${STAGING}/katex"
+readonly _src=${ROOT}/node_modules/katex/dist
+readonly _dest="${STAGING}/katex"
 
 # KaTeX provides three versions of each of their font files: TrueType, WOFF,
 # and WOFF2. WOFF2 support is available as of (iOS) Safari 10 and Chrome 36.
-_katex_font_type=woff2
+readonly _katex_font_type=woff2
 
 z_rsync "${_src}/" "${_dest}" <<EOF
 # Include these two files.