Order of CSS custom variable declarations is inconsistent

[NikaBuligini]

Lightningcss is ordering CSS custom variables inconsistently.

Even if I don't make changes in the source file I might get a different output. This is a problem since I'm using contenthash in filenames in order to let users keep using cached version of the file when there are no changes in the content. So, change in order effectivelly invalidates cache for users.

How to reproduce:

• Go to lightningcss playground
• Uncheck minify to easily see the difference in the output

• Set the following as input:

  .my-class-name {
  --foo: 1.1em;
  --bar: currentcolor;
  
  all: unset;
  }

• Keep changing and undoing the input to trigger the compilation (you can also add letters to one of the variable names)
• You will see that --foo and --bar variables are getting ordered differently

https://github.com/user-attachments/assets/878a2f90-0e82-4c9c-8b11-15fc31ce435e

If you remove all: unset or move it above variable declarations then the issue with the order can't be reproduced anymore.