fetchurl: builder.sh: handle urls as a Bash array

Clean up leftover for commit cd13136f03
("fetchurl: use __structuredAttrs = true and pass curlOptsList directly")

Continue the work of commit 23236b331d
("fetchurl: fix handling of fallback URLs"),
addressing a Bash array re-assignment quirk:
when assigning a Bash array variable as if it were a plain variable,
the value goes to the first element,
and the rest of the array stays the same.

```console
$ foo=(a b)
$ declare -p foo
declare -a foo=([0]="a" [1]="b")
$ foo="c d"
$ declare -p foo
declare -a foo=([0]="c d" [1]="b")
```

Don't rewrite the `${urls[@]}` with resolved URLs,
but hold them with `${resolvedUrls[@]}` instead.

Co-authored-by: Matt Sturgeon <matt@sturgeon.me.uk>
Co-authored-by: Wolfgang Walther <walther@technowledgy.de>

pkgs/build-support/fetchurl/builder.sh

@@ -124,10 +124,10 @@ tryHashedMirrors() {
 # URL list may contain ?. No glob expansion for that, please
 set -o noglob
 
-urls2=
+resolvedUrls=()
 for url in "${urls[@]}"; do
     if test "${url:0:9}" != "mirror://"; then
-        urls2="$urls2 $url"
+        resolvedUrls+=("$url")
     else
         url2="${url:9}"; echo "${url2/\// }" > split; read site fileName < split
         #varName="mirror_$site"
@@ -142,18 +142,17 @@ for url in "${urls[@]}"; do
             if test -n "${!varName}"; then mirrors="${!varName}"; fi
 
             for url3 in $mirrors; do
-                urls2="$urls2 $url3$fileName";
+                resolvedUrls+=("$url3$fileName");
             done
         fi
     fi
 done
-urls="$urls2"
 
 # Restore globbing settings
 set +o noglob
 
 if test -n "$showURLs"; then
-    echo "$urls" > $out
+    echo "${resolvedUrls[*]}" > $out
     exit 0
 fi
 
@@ -165,7 +164,7 @@ fi
 set -o noglob
 
 success=
-for url in $urls; do
+for url in "${resolvedUrls[@]}"; do
     if [ -z "$postFetch" ]; then
        case "$url" in
            https://github.com/*/archive/*)