Clean RUST_TARGET_DIR when soroban rlibs change

[graydon]

It seems -- I'll use "seems" here because I am not 100% certain but this is congruent with my observations of errors so far -- that rustc and/or cargo don't do a good job of tracking the dependency of librust_stellar_core.a on the manually-provided soroban rlibs: that recompiling the soroban rlibs causes us to trigger a rebuild of librust_stellar_core.a but cargo and/or rustc frequently decides that "nothing changed in its dependencies" and so doesn't actually do any recompiling. Which means we can wind up with stale soroban bits in librust_stellar_core.a.

(It also "seems" like the stamp-file time tracking based on touch isn't reliably triggering a rebuild; this seems like it should never be true since touch alters mtime which is all that make tracks, but I think I observed such a failure during debugging and it's harmless to remove-and-recreate the stamp-file just to be sure.)