configure: repair the check if argv can be written to

configure: repair the check if argv can be written to

Due to bad escaping of the test code, the test wouldn't build and thus
result in a negative test result, which would lead to the unconditional
assumption that overwriting the arguments doesn't work and thus curl
would never hide credentials given in the command line, even when it
would otherwise be possible.

Regression from commit 2d4c2152c (7.60.0)

Reported-by: huzunhao on github
Fixes #5470
Closes #5471