Use target.'cfg(all())'.rustflags in template

`target.'cfg(all())'.linker` is apparently a recent addition:
rust-lang/cargo#12535

Author: smoelius

internal/template/.cargo/config.toml

@@ -1,2 +1,6 @@
 [target.'cfg(all())']
-linker = "dylint-link"
+rustflags = ["-C", "linker=dylint-link"]
+
+# For Rust versions 1.74.0 and onward, the following alternative can be used
+# (see https://github.com/rust-lang/cargo/pull/12535):
+# linker = "dylint-link"