makepkg: Run verify in SRCDEST, not always startdir

`verify()` verifies downloaded sources before we link or copy them into
`$srcdir`. For this reason it runs in `$startdir`. However, sources are
actually downloaded into `$SRCDEST`, which merely defaults to
`$startdir`.

doc/PKGBUILD.5.asciidoc

@@ -342,7 +342,8 @@ function.
 	An optional `verify()` function can be specified to implement arbitrary
 	source authentication. The function should return a non-zero exit code when
 	verification fails. This function is run before sources are extracted.
-	This function is run inside `$startdir`.
+	This function is run inside `$SRCDEST`, if set, otherwise the same
+	directory as the PKGBUILD.
 
 *prepare() Function*::
 	An optional `prepare()` function can be specified in which operations to

scripts/makepkg.sh.in

@@ -450,7 +450,7 @@ run_function() {
 }
 
 run_verify() {
-	run_function_safe "verify" "$startdir"
+	run_function_safe "verify" "$SRCDEST"
 }
 
 run_prepare() {