Hack around proc-macro2 version

Done to make the minimal (crate) version CI happy.

Cargo.toml

@@ -64,3 +64,11 @@ serde_test = "1.0"
 sval = { version = "2.1" }
 sval_derive = { version = "2.1" }
 value-bag = { version = "1.4", features = ["test"] }
+
+# NOTE: log doesn't actually depent on this crate. However our dependencies,
+# serde and sval, dependent on version 1.0 of the crate, which has problem fixed
+# in 1.0.60, specifically in the following commit
+# https://github.com/dtolnay/proc-macro2/commit/e31d61910049e097afdd3d27c37786309082bdcb.
+# By defining the crate as direct dependency we can increase it's minimal
+# version making the minimal (crate) version CI happy.
+proc-macro2 = { version = "1.0.63", default-features = false }