tests: Adjust ‘guix build -P1’ test to new packages.

Fixes a bug introduced in 794e079437
whereby the grep patterns used in this test would also match
/gnu/store/…-rust-libgit2-sys-0.17.0+1.8.1.drv, thus getting 5 matches
instead of 4.

* tests/guix-build.sh: Refine grep patterns for ‘guix build -P1’ test.

Reported-by: Janneke Nieuwenhuizen <janneke@gnu.org>
Change-Id: Iae35e6ee44b8eca0ef66c1d8ab7f9ff65a59d8d0

tests/guix-build.sh

@@ -199,7 +199,7 @@ test `guix build -D hello -d \
 # Building the dependents.
 test `guix build -P1 libgit2 -P1 libssh -d \
       | grep -e 'guile-git.*\.drv$' -e 'guile-ssh.*\.drv$' \
-             -e 'libgit2.*\.drv$' -e 'libssh.*\.drv$' \
+             -e 'libgit2-[0-9].*\.drv$' -e 'libssh-[0-9].*\.drv$' \
       | wc -l` -eq 4
 
 # Unbound variable in thunked field.