reproducible-builds.sh: fatal: += not allowed in assignment

take in account undefined case
assign and export in separate step
https://github.com/koalaman/shellcheck/wiki/SC2155

pkgs/build-support/setup-hooks/reproducible-builds.sh

@@ -1,9 +1,13 @@
+# shellcheck disable=SC2148
 # Use the last part of the out path as hash input for the build.
 # This should ensure that it is deterministic across rebuilds of the same
 # derivation and not easily collide with other builds.
 # We also truncate the hash so that it cannot cause reference cycles.
-export NIX_CFLAGS_COMPILE+=" -frandom-seed=$(
+# NIX_CFLAGS_COMPILE might not have been defined before
+NIX_CFLAGS_COMPILE="${NIX_CFLAGS_COMPILE:-} -frandom-seed=$(
+    # shellcheck disable=SC2154
     outbase="${out##*/}"
     randomseed="${outbase:0:10}"
-    echo $randomseed
+    echo "$randomseed"
 )"
+export NIX_CFLAGS_COMPILE