packaging: enable rust extensions on centos

Test on CentOS 7, repository with ~170000 tracked files,
    no untracked files:
10 runs with this enabled:
 -- Run  #0 time: 0.6519973278045654
 -- Run  #1 time: 0.6933724880218506
 -- Run  #2 time: 0.7512078285217285
 -- Run  #3 time: 0.7517638206481934
 -- Run  #4 time: 0.5966529846191406
 -- Run  #5 time: 0.5960886478424072
 -- Run  #6 time: 0.5940573215484619
 -- Run  #7 time: 0.5963726043701172
 -- Run  #8 time: 0.6048009395599365
 -- Run  #9 time: 0.603604793548584

10 runs without this enabled:
 -- Run  #0 time: 2.127584457397461
 -- Run  #1 time: 2.066192865371704
 -- Run  #2 time: 2.0831892490386963
 -- Run  #3 time: 2.077716588973999
 -- Run  #4 time: 2.07608962059021
 -- Run  #5 time: 2.072899341583252
 -- Run  #6 time: 2.094369888305664
 -- Run  #7 time: 2.067504644393921
 -- Run  #8 time: 2.069610834121704
 -- Run  #9 time: 2.0567898750305176

Differential Revision: https://phab.mercurial-scm.org/D9294

contrib/packaging/docker/centos7

@@ -15,3 +15,6 b' RUN yum install -y \\'
 
 # For creating repo meta data
 RUN yum install -y createrepo
+
+# For rust extensions
+RUN yum install -y cargo

contrib/packaging/docker/centos8

@@ -13,3 +13,6 b' RUN yum install -y \\'
 
 # For creating repo meta data
 RUN yum install -y createrepo
+
+# For rust extensions
+RUN yum install -y cargo

contrib/packaging/mercurial.spec

@@ -110,14 +110,14 b' cd %{docutilsname}'
 LD_LIBRARY_PATH=$PYPATH $PYPATH/python setup.py install --root="$RPM_BUILD_ROOT"
 cd -
 
-PATH=$PYPATH:$PATH LD_LIBRARY_PATH=$PYPATH make install PYTHON=%{pythonexe} DESTDIR=$RPM_BUILD_ROOT PREFIX=%{hgpyprefix} MANDIR=%{_mandir}
+PATH=$PYPATH:$PATH LD_LIBRARY_PATH=$PYPATH make install PYTHON=%{pythonexe} DESTDIR=$RPM_BUILD_ROOT PREFIX=%{hgpyprefix} MANDIR=%{_mandir} PURE="--rust"
 mkdir -p $RPM_BUILD_ROOT%{_bindir}
 ( cd $RPM_BUILD_ROOT%{_bindir}/ && ln -s ../..%{hgpyprefix}/bin/hg . )
 ( cd $RPM_BUILD_ROOT%{_bindir}/ && ln -s ../..%{hgpyprefix}/bin/python2.? %{pythonhg} )
 
 %else
 
-make install PYTHON=%{pythonexe} DESTDIR=$RPM_BUILD_ROOT PREFIX=%{_prefix} MANDIR=%{_mandir}
+make install PYTHON=%{pythonexe} DESTDIR=$RPM_BUILD_ROOT PREFIX=%{_prefix} MANDIR=%{_mandir} PURE="--rust"
 
 %endif