Remove extra parentheses

[bugorz]

Note to reviewers

• It could be just me, but it seems like the extra parentheses made cd $(RISCV_TESTS_DIR); ./configure ineffective. Therefore, the Makefile in tests/riscv-tests wasn't generated when I built a fresh checkout of semu.
• Tested on macOS Monterey 12.4. I'll try to test it on CentOS tomorrow.
• Thanks for making the build modularized :)

Tests

• Here is a fresh checkout of the semu repo

  
  ➜  semu git:(master) git diff
  ➜  semu git:(master) git s
  On branch master
  Your branch is up to date with 'origin/master'.

nothing to commit, working tree clean ➜ semu git:(master) make ENABLE_RISCV_TESTS=1 clean run-tests
CC semu.o CC tests/isa-test.o

(omitted)

Building riscv-tests... riscv64-unknown-elf-objcopy: 'tests/riscv-tests/isa/rv64ui-p-add': No such file make: *** [tests/riscv-tests-data/rv64ui-p-add] Error 1

 - After I removed the parentheses
```shell
➜  semu git:(master) vi mk/riscv-tests.mk 
➜  semu git:(master) ✗ git s                                    
On branch master
Your branch is up to date with 'origin/master'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
    modified:   mk/riscv-tests.mk

no changes added to commit (use "git add" and/or "git commit -a")
➜  semu git:(master) ✗ make ENABLE_RISCV_TESTS=1 clean run-tests
  CC    semu.o
  CC    tests/isa-test.o

(omitted)

Building riscv-tests...
configure: WARNING: using cross tools not prefixed with host triplet
[==========] Running 70 test(s) from riscv-tests.

(omitted)

[==========] 70 test(s) from riscv-tests ran.
[  PASSED  ] 54 test(s).

(omitted)

[jserv]

Thank @bugorz for contributing! I felt ashamed on the stupid mistake.

[bugorz]

@jserv No worries, this one is sneaky!