STYLE: Call write once instead of looping

Call `write` once instead of looping by concatenating all strings that
need to be written.

Avoid using `writelines` as `ImageOpener` objects do not have that
attribute.

Fixes:
```
-                for val in bvals:
-                    fid.write(f'{val} ')
+                fid.writelines(f'{val} ' for val in bvals)
```

and other similar errors raised for example in:
https://github.com/nipy/nibabel/actions/runs/16092302666/job/45410655457?pr=1422#step:7:23

Documentation:
https://docs.astral.sh/ruff/rules/for-loop-writes/

Author: jhlegarreta

nibabel/cmdline/parrec2nii.py

@@ -358,8 +358,7 @@ def proc_file(infile, opts):
             )
             with open(basefilename + '.bvals', 'w') as fid:
                 # np.savetxt could do this, but it's just a loop anyway
-                for val in bvals:
-                    fid.write(f'{val} ')
+                fid.write(' '.join(str(val) for val in bvals) + ' ')
                 fid.write('\n')
         else:
             verbose('Writing .bvals and .bvecs files')
@@ -369,13 +368,11 @@ def proc_file(infile, opts):
             bvecs = apply_affine(bv_reorient, bvecs)
             with open(basefilename + '.bvals', 'w') as fid:
                 # np.savetxt could do this, but it's just a loop anyway
-                for val in bvals:
-                    fid.write(f'{val} ')
+                fid.write(' '.join(str(val) for val in bvals) + ' ')
                 fid.write('\n')
             with open(basefilename + '.bvecs', 'w') as fid:
                 for row in bvecs.T:
-                    for val in row:
-                        fid.write(f'{val} ')
+                    fid.write(' '.join(str(val) for val in bvals) + ' ')
                     fid.write('\n')
 
     # export data labels varying along the 4th dimensions if requested