chore: bump savvy from 0.6.7 to 0.6.8 in /src/rust (#310)

Author: dependabot[bot]

src/rust/Cargo.lock

@@ -10,7 +10,7 @@ crate-type = ['staticlib']
 name = "prqlr"
 
 [dependencies]
-savvy = "0.6.7"
+savvy = "0.6.8"
 # prqlc= { version = "0.11.2", default-features = false }
 # prqlc 0.11.2 is not compatible with Rust 1.65, so a slightly modified version is installed
 # See https://github.com/PRQL/prql/pull/4001