Pin memchr in our release dependency list due to core2 using it

We're working with rust-bitcoin to remove the `core2` dependency
at rust-bitcoin/rust-bitcoin#2066 but until
that lands and we can upgrade rust-bitcoin we're stuck with it. In
the mean time, we should still pass our MSRV tests.

Author: TheBlueMatt

ci/ci-tests.sh

@@ -18,6 +18,11 @@ function PIN_RELEASE_DEPS {
 	# The serde_json crate switched to Rust edition 2021 starting with v1.0.101, i.e., has MSRV of 1.56
 	[ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p serde_json --precise "1.0.100" --verbose
 
+	# The memchr crate switched to an MSRV of 1.60 starting with v2.6.0
+	# This is currently only a release dependency via core2, which we intend to work with
+	# rust-bitcoin to remove soon.
+	[ "$RUSTC_MINOR_VERSION" -lt 60 ] && cargo update -p memchr --precise "2.5.0" --verbose
+
 	return 0 # Don't fail the script if our rustc is higher than the last check
 }