feat!: fix ccache overlay and enable by default, add ismpc

[arntanguy]

• Fixes use of ccache, in particular with buildRosPackages
• Fixes overlays to always use the final overlay
• Enables ccache by default (let's see)

In addition: Adds ismpc-walking and related pkgs

After a bit of use:

Cacheable calls:    4475 / 6827 (65.55%)
  Hits:             1516 / 4475 (33.88%)
    Direct:         1000 / 1516 (65.96%)
    Preprocessed:    516 / 1516 (34.04%)
  Misses:           2959 / 4475 (66.12%)
Uncacheable calls:  2352 / 6827 (34.45%)
Local storage:
  Cache size (GiB):  2.3 /  5.0 (45.09%)
  Cleanups:          211
  Hits:             1516 / 4475 (33.88%)
  Misses:           2959 / 4475 (66.12%)