Remove -j argument from C runtime make invocation

`-j` cannot be used in the juvix_c make invocation because it causes
an error in the 'Build Linux static binary workflow'

```
cd runtime && make
make[1]: Entering directory '/__w/juvix/juvix/runtime'
cd c && make -j 4 -s
make[3]: *** No rule to make target '_build.wasm32-wasi/src/juvix/arch/wasi.o', needed by 'all'.  Stop.
make[3]: *** No rule to make target '_build.wasm32-wasi-debug/src/juvix/arch/wasi.o', needed by 'all'.  Stop.
make[2]: *** [Makefile:20: wasm32-wasi] Error 2
make[2]: *** Waiting for unfinished jobs....
make[2]: *** [Makefile:35: wasm32-wasi-debug] Error 2
_build.native64/src/juvix/opts.d:1: *** recipe commences before first target.  Stop.
make[2]: *** [Makefile:26: native64] Error 2
_build.native64-debug/src/juvix/opts.d:1: *** recipe commences before first target.  Stop.
make[2]: *** [Makefile:41: native64-debug] Error 2
make[1]: *** [Makefile:9: juvix_c] Error 2
make[1]: Leaving directory '/__w/juvix/juvix/runtime'
make: *** [Makefile:206: runtime] Error 2
```

runtime/Makefile

@@ -4,9 +4,11 @@ export
 .PHONY: all
 all: juvix_c juvix_rust
 
+# 2024-09-20: `-j` cannot be used in the juvix_c make invocation because it causes
+# an error in the 'Build Linux static binary workflow'.
 .PHONY: juvix_c
 juvix_c:
-	cd c && $(MAKE) -j 4 -s
+	cd c && $(MAKE) -s
 
 .PHONY: juvix_rust
 juvix_rust: