cargo: explicitly configure musl to be statically linked

It looks like the musl target will, at some point, default to be
dynamically linked. This config knob should make it so that it's always
statically linked.

Ref rust-lang/compiler-team#422
Ref rust-lang/compiler-team#422 (comment)

.cargo/config.toml

@@ -6,3 +6,16 @@
 rustflags = ["-C", "target-feature=+crt-static"]
 [target.i686-pc-windows-msvc]
 rustflags = ["-C", "target-feature=+crt-static"]
+
+# Do the same for MUSL targets. At the time of writing (2023-10-23), this is
+# the default. But the plan is for the default to change to dynamic linking.
+# The whole point of MUSL with respect to ripgrep is to create a fully
+# statically linked executable.
+#
+# See: https://github.com/rust-lang/compiler-team/issues/422
+# See: https://github.com/rust-lang/compiler-team/issues/422#issuecomment-812135847
+[target.x86_64-unknown-linux-musl]
+rustflags = [
+  "-C", "target-feature=+crt-static",
+  "-C", "link-self-contained=yes",
+]