Fix the nightly CI workflow.

The `toml` crate recently made a release that no longer builds
with Rust 1.65. This commit pins that crate along with a few
of its dependencies to preserve the ability of this project
to build on the "oldest supported" version of Rust.

mpdpopm/Cargo.toml.in

@@ -35,8 +35,10 @@ os_str_bytes = "2.3.1"
 pin-project = "1.0.8"
 regex = "1.3.6"
 serde = { version = "1.0", features = ["derive"] }
+serde_spanned = "=0.6.3" # Pin to preserve the ability to build with rust 1.65
 serde-lexpr = "0.1.1"
 toml = "=0.7.6" # Pin to preserve the ability to build with rust 1.65
+toml_datetime = "=0.6.3" # Pin to preserve the ability to build with rust 1.65
 toml_edit = "=0.19.12" # Pin to preserve the ability to build with rust 1.65
 tokio = { version = "1.15.0", features = ["io-util", "macros", "net", "process", "rt-multi-thread", "signal", "time"] }