meson: call find_program() once and reuse the variable everywhere

meson: call find_program() once and reuse the variable everywhere

Meson 0.58 has gotten quite bad with emitting a message every time
a quoted command is used:
Program /home/zbyszek/src/systemd-work/tools/meson-make-symlink.sh found: YES (/home/zbyszek/src/systemd-work/tools/meson-make-symlink.sh)
Program sh found: YES (/usr/bin/sh)
Program sh found: YES (/usr/bin/sh)
Program sh found: YES (/usr/bin/sh)
Program sh found: YES (/usr/bin/sh)
Program sh found: YES (/usr/bin/sh)
Program sh found: YES (/usr/bin/sh)
Program xsltproc found: YES (/usr/bin/xsltproc)
Configuring custom-entities.ent using configuration
Message: Skipping bootctl.1 because ENABLE_EFI is false
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Message: Skipping journal-remote.conf.5 because HAVE_MICROHTTPD is false
Message: Skipping journal-upload.conf.5 because HAVE_MICROHTTPD is false
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Message: Skipping loader.conf.5 because ENABLE_EFI is false
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
Program ln found: YES (/usr/bin/ln)
...

Let's suffer one message only for each command. Hopefully we can silence
even this when https://github.com/mesonbuild/meson/issues/8642 is
resolved.