depends: Use base system's sha256sum utility

On FreeBSD, the `shasum` utility is provided by the `perl5` port, which
is not part of the base system and must be installed separately.
Note that this requirement is currently not documented in
`depends/README.md`.

This change switches to using the `sha256sum` utility, which is included
in the base system.

depends/builders/freebsd.mk

@@ -1,5 +1,5 @@
 build_freebsd_CC=clang
 build_freebsd_CXX=clang++
 
-build_freebsd_SHA256SUM = shasum -a 256
+build_freebsd_SHA256SUM = sha256sum
 build_freebsd_DOWNLOAD = curl --location --fail --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o