Trac #32836: Work around clobbering of PATH on initialization of libsingular from system singular

I'm pretty sure this is a new problem, otherwise I would have hit it
with lcalc-2.x before. These tests were done on a clean 9.5.beta5 build.

When I start sage, my `PATH` looks correct, in that `SAGE_LOCAL/bin`
comes before the usual system paths (in particular `/usr/bin`):

{{{
sage: os.environ["PATH"].split(":")
['/home/mjo/src/sage.git/build/bin',
 '/home/mjo/src/sage.git/src/bin',
 '/home/mjo/src/sage.git/local/var/lib/sage/venv-python3.9/bin',
 '/home/mjo/src/sage.git/local/bin',
 '/usr/local/sbin',
 '/usr/local/bin',
 '/usr/sbin',
 '/usr/bin',
 '/sbin',
 '/bin',
 '/opt/bin',
 '/usr/lib/llvm/12/bin',
 '/home/mjo/bin',
 '/home/mjo/bin']
}}}

And if I use `shutil.which` to find the "lcalc" program, it finds the
right one:

{{{
sage: from shutil import which
sage: which("lcalc")
'/home/mjo/src/sage.git/local/bin/lcalc'
sage: os.system(which("lcalc") + " --version")
lcalc 1.22 July 14, 2009
0
}}}

But... that's not the one that sage itself wants to use:

{{{
sage: os.system("lcalc --version")
lcalc 2.0.3
0
sage: from sage.lfunctions.lcalc import LCalc
sage: lc = LCalc()
sage: lc("--version")
'lcalc 2.0.3'
}}}

This is causing all doctests that run "lcalc" to fail, because they're
picking up lcalc-2.x from `/usr/bin` instead of the one in
`SAGE_LOCAL/bin` that the SPKG installed.

URL: https://trac.sagemath.org/32836
Reported by: mjo
Ticket author(s): Michael Orlitzky
Reviewer(s): Matthias Koeppe

Author: Release Manager

src/sage/libs/singular/singular.pyx

@@ -803,8 +803,11 @@ cdef init_libsingular():
 
     error_messages = []
 
-# call the init routine
+# Save/restore the PATH because libSingular clobbers it:
+# https://github.com/Singular/Singular/issues/1119
+saved_PATH = os.environ["PATH"]
 init_libsingular()
+os.environ["PATH"] = saved_PATH
 
 cdef void libsingular_error_callback(const_char_ptr s):
     _s = char_to_str(s)