[skip ci] Rollback master branch to 0.25.0

cargo-release automatically incremented the master branch's version
post-release.  I don't like that, because it makes it harder for
consumers, especially indirect consumers, to use a `[patch.crates-io]`
section in their Cargo.toml files.

Also, configure cargo-release not to do this again in the future.

Cargo.toml

@@ -2,7 +2,7 @@
 name        = "nix"
 description = "Rust friendly bindings to *nix APIs"
 edition     = "2018"
-version     = "0.25.1-alpha.0"
+version     = "0.25.0"
 rust-version = "1.46"
 authors     = ["The nix-rust Project Developers"]
 repository  = "https://github.com/nix-rust/nix"

release.toml

@@ -1,3 +1,4 @@
+dev-version = false
 pre-release-replacements = [
 	{ file="CHANGELOG.md", search="Unreleased", replace="{{version}}" },
 	{ file="CHANGELOG.md", search="ReleaseDate", replace="{{date}}" }