Make timing based performance tests opt in via RAKUDO_RUN_TIMING_TESTS

It's easier to tell a hand full of core developers to set an environment
variable to opt into performance regression tests than to tell an unknown
number of unknown people to opt out of them for automated builds.

Fixes #3065

appveyor.yml

@@ -49,7 +49,6 @@ install:
   - SET PATH=C:\strawberry\c\bin;C:\strawberry\perl\site\bin;C:\strawberry\perl\bin;%PATH%
   - ECHO %RAKUDO_OPTIONS%
   - CD %APPVEYOR_BUILD_FOLDER%
-  - SET RAKUDO_SKIP_TIMING_TESTS=1
 
 # Testing
 test_script:

t/08-performance/99-misc.t

@@ -1,6 +1,6 @@
 use Test;
 
-my $skip = %*ENV<RAKUDO_SKIP_TIMING_TESTS> ?? 1 !! 0;
+my $skip = %*ENV<RAKUDO_RUN_TIMING_TESTS> ?? 0 !! 3;
 
 plan 4 - $skip;
 
@@ -15,7 +15,7 @@ plan 4 - $skip;
     is-deeply $f cmp ($ = $f), Same, 'eqaddr optimization for cmp exists'
 }
 
-{
+unless $skip {
     my int @a = ^2_000_000;
     my $then = now;
     my $result1 = @a.sum;