Switch to runtime flag for enabling indexes

Makes it easier to test the feature:

```
$ cargo run --  --experimental-indexes
Limbo v0.0.22
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database
limbo> CREATE TABLE t(x);
limbo> CREATE INDEX t_idx ON t(x);
limbo> DROP INDEX t_idx;
```

.github/workflows/long_fuzz_tests_btree.yml

@@ -29,7 +29,7 @@ jobs:
       env:
         RUST_BACKTRACE: 1
     - name: Run ignored long tests with index
-      run: cargo test --features index_experimental -- --ignored fuzz_long
+      run: cargo test -- --ignored fuzz_long
       env:
         RUST_BACKTRACE: 1
 

.github/workflows/rust.yml

@@ -41,12 +41,6 @@ jobs:
         RUST_LOG: ${{ runner.debug && 'limbo_core::storage=trace' || '' }}
       run: cargo test --verbose
       timeout-minutes: 20
-    - name: Tests with indexes
-      env:
-        RUST_LOG: ${{ runner.debug && 'limbo_core::storage=trace' || '' }}
-      run: cargo test --verbose --features index_experimental
-      timeout-minutes: 20
-
 
   clippy:
     runs-on: blacksmith-4vcpu-ubuntu-2404