libsql-sqlite3: Pin psm version in extension tests (#2221)

ar_archive_writer v0.5.1 pulled by latest psm uses let-chains which
require Rust 1.87+. Since the rust_suite Cargo.lock is not tracked, CI
resolves to the latest version which breaks the build.

Author: penberg

libsql-sqlite3/test/rust_suite/Cargo.toml

@@ -18,6 +18,9 @@ rustc-hash = "1"
 home = { version = "=0.5.9" }
 which = "=4.4.0"
 
+[dev-dependencies.psm]
+version = "=0.1.21"
+
 [features]
 default = []
 udf = []