.cargo/config.toml: define linker in dedicated field

this has been introduced in cargo 1.74, thus we no longer need to define
it in the rustflags.

Author: rursprung

examples/stm32f4-event-printer/.cargo/config.toml

@@ -1,9 +1,12 @@
 [target.thumbv7em-none-eabihf]
 runner = 'probe-run --chip STM32F401RE'
+
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+linker = "flip-link"
+
 rustflags = [
     "-C", "link-arg=-Tlink.x",
     "-C", "link-arg=-Tdefmt.x",
-    "-C", "linker=flip-link",
 ]
 
 [build]