Don't fail if env-vars cannot be written to

env-vars is a debugging aid, see
3e5dbb2433
for a rationale for this change.

pkgs/build-support/builder-defs/builder-defs.nix

@@ -336,7 +336,7 @@ let inherit (builtins) head tail trace; in
 
         doDump = n: noDepEntry "echo Dump number ${n}; set";
 
-	saveEnv = noDepEntry ''export > $TMP/env-vars'';
+	saveEnv = noDepEntry ''export > "$TMP/env-vars" || true'';
 
 	doDumpBuildInputs = noDepEntry (''
 	  echo "${toString realBuildInputs}"

pkgs/stdenv/generic/setup.sh

@@ -450,7 +450,7 @@ substituteAllInPlace() {
 # the environment used for building.
 dumpVars() {
     if [ "$noDumpEnvVars" != 1 ]; then
-        export > "$NIX_BUILD_TOP/env-vars"
+        export > "$NIX_BUILD_TOP/env-vars" || true
     fi
 }