generic/setup.sh: allow clobbering env-vars file

If the option ‘noclobber’ is set in Bash, we get an error when we
clobber an already existing env-vars. This is an okay error to ignore,
so just >| instead. Note that >| is NOT a Bashism[[1]].

Fixes #79651

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_02

pkgs/stdenv/generic/setup.sh

@@ -779,7 +779,7 @@ substituteAllInPlace() {
 # the environment used for building.
 dumpVars() {
     if [ "${noDumpEnvVars:-0}" != 1 ]; then
-        export > "$NIX_BUILD_TOP/env-vars" || true
+        export >| "$NIX_BUILD_TOP/env-vars" || true
     fi
 }