fix(ci): bust stale nightly _build cache holding pre-OTP-29 meck

[Taure]

Problem

Nightly Property Tests has failed every night (issues #61/#65/#68/#69) with a compile error in meck, not a property test:

===> Compiling _build/test/lib/meck/src/meck_matcher.erl failed
 76 │  (catch erlang:apply(hamcrest, assert_that, [Value, HamcrestMatcher])) == true;
    │   ╰── 'catch ...' is deprecated; please use 'try ... catch ... end' instead.
##[error]Process completed with exit code 1.

Root cause

Same as widgrensit/asobi#155. meck is pulled transitively via the asobi git dep and, like proper, is a test-profile dep that rebar3 never writes to rebar.lock. The nightly cache key was hashFiles('rebar.lock'), so it never busted; restore-keys: nightly-prop- restored an old _build holding meck 0.9.x (bare catch, fatal under OTP 29), which rebar3 recompiled instead of fetching the fixed 1.2.0.

Fix

• Hash rebar.config in the cache key.
• Bump the prefix to nightly-prop-v2- to retire the poisoned caches.

Pairs with widgrensit/asobi#155.