debuild intentionally sanitizes the environment for security reasons, according to the manual page:
As environment variables can affect the building of a package, often unintentionally, debuild sanitises the environment by removing all environment variables except for TERM, HOME, LOGNAME, GNUPGHOME, PGPPATH, GPG_AGENT_INFO, GPG_TTY, DBUS_SESSION_BUS_ADDRESS, FAKEROOTKEY, DEBEMAIL, DEB_*, the (C, CPP, CXX, LD and F)FLAGS variables and their _APPEND counterparts and the locale variables LANG and LC_*. TERM is set to dumb if it is unset, and PATH is set to /usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11.
In general, you should not build a Debian package using software that is not also a Debian package because it's impossible to produce a reproducible build and for others to build the package outside of your environment. Therefore, the best solution is to use the Debian rustc package, use a newer version from Debian unstable, or build a newer rustc if you require one. If you are sure that you really want to do this anyway and thoroughly understand the consequences, there are configuration options to adjust the PATH environment variable.