build: disable crt-static for musl targets

Alpine has no libxkbcommon.a, so static linking fails at ld.

.cargo/config.toml

@@ -0,0 +1,11 @@
+# Alpine/musl hosts: rustup's *-linux-musl targets default to crt-static, which
+# tries to statically link system libraries as well. Alpine ships libxkbcommon.so
+# but no libxkbcommon.a, so the final link fails with:
+#     ld: cannot find -lxkbcommon
+#     ld: have you installed the static version of the xkbcommon library ?
+# Link dynamically instead, which is what a binary running on the host wants.
+[target.aarch64-unknown-linux-musl]
+rustflags = ["-C", "target-feature=-crt-static"]
+
+[target.x86_64-unknown-linux-musl]
+rustflags = ["-C", "target-feature=-crt-static"]