logo: Fix nondeterminism in ImageMagick step

The ImageMagick `convert` command, introduced in b13cf4e,
apparently inserts the current time into the file.  That makes the
build not reproducible.

Fix that using the "strip-nondeterminism" tool from the
Reproducible Builds project:
  https://reproducible-builds.org/docs/timestamps/

The resulting change to the affected file can be seen using `sng`,
and purely removes a pair of timestamps:

$ f=ios/Runner/Assets.xcassets/AppIcon.appiconset/[email protected] \
  && diff -U3 <(git show @:"$f" | sng) \
              <(<"$f" sng)
--- /dev/fd/63	2025-01-06 21:11:42.422900540 -0800
+++ /dev/fd/62	2025-01-06 21:11:42.426900539 -0800
@@ -13,14 +13,6 @@
 bKGD {red: 255;  green: 255;  blue: 255;}
 pHYs {xpixels: 161260; ypixels: 161260; per: meter;}  # (4096 dpi)
 tEXt {
-    keyword: "date:create";
-    text: "2023-11-27T17:11:13-08:00";
-}
-tEXt {
-    keyword: "date:modify";
-    text: "2023-11-27T17:11:13-08:00";
-}
-tEXt {
     keyword: "Software";
     text: "www.inkscape.org";
 }

Author: gnprice

ios/Runner/Assets.xcassets/AppIcon.appiconset/[email protected]

@@ -44,6 +44,9 @@ cwebp -version >/dev/null 2>&1 \
 jq --version >/dev/null 2>&1 \
     || die "Need jq -- try 'apt install jq'."
 
+strip-nondeterminism --version >/dev/null 2>&1 \
+    || die "Need strip-nondeterminism -- try 'apt install strip-nondeterminism'."
+
 
 # This should point to a zulip.git worktree.
 zulip_root="${root_dir%/*}"/zulip
@@ -109,6 +112,7 @@ make_ios_app_icon() {
     mv "${iconset}"/Icon-1024x1024@1x{,.1}.png
     convert "${iconset}"/[email protected] \
       -alpha deactivate "${iconset}"/[email protected]
+    strip-nondeterminism "${iconset}"/[email protected]
     rm -f "${iconset}"/[email protected]
 
     # From "Spotlight, Settings, and Notification Icons"