tox.ini: Initialize the BOOTSTRAP variable

.gitignore

@@ -107,3 +107,4 @@ gitlab-build-docker.log
 
 /.tox
 /prefix
+worktree*

tox.ini

@@ -107,6 +107,8 @@ passenv =
     local-direct: *
 
 setenv =
+    # By default, we bootstrap using autotools and do not allow downloads of the configure tarball.
+    BOOTSTRAP=./bootstrap
     # For https downloads from upstream_url listed in checksums.ini, do not
     # check certificates, to avoid problems on Docker images with outdated
     # certificates and with system python3 on macOS (#29418)
@@ -190,7 +192,7 @@ setenv =
     fedora-32:        BASE_TAG=32
     #
     # https://hub.docker.com/_/centos
-    # centos-6 only has autoconf 2.63 -- too old for bootstrap
+    # centos-6 only has autoconf 2.63 -- too old for bootstrap; download configure tarball instead.
     #
     centos:         SYSTEM=fedora
     centos:         BASE_IMAGE=centos
@@ -408,7 +410,7 @@ commands =
     docker:        done'
     # pathpy checksuite needs tox. #28728: gap fails its test suite.
     # linbox/cysignals testsuites fail.  ppl takes very long.
-    local:         bash -c 'export PATH={env:PATH} && {env:SETENV} && {env:BOOTSTRAP:./bootstrap} && ./configure --prefix={envdir}/local {env:CONFIGURE_ARGS} && make -k V=0 base-toolchain && make -k V=0 SAGE_SPKG="sage-spkg -y -o" SAGE_CHECK=warn SAGE_CHECK_PACKAGES="!cython,!r,!python3,!nose,!pathpy,!gap,!cysignals,!linbox,!git,!ppl,!cmake,!networkx,!symengine_py" {env:TARGETS_PRE:} {posargs:build} && (make -k V=0 SAGE_SPKG="sage-spkg -y -o" SAGE_CHECK=warn SAGE_CHECK_PACKAGES="!cython,!r,!python3,!nose,!pathpy,!gap,!cysignals,!linbox,!git,!ppl,!cmake" {env:TARGETS_OPTIONAL:} || echo "(error ignored)" ) '
+    local:         bash -c 'export PATH={env:PATH} && {env:SETENV} && {env:BOOTSTRAP} && ./configure --prefix={envdir}/local {env:CONFIGURE_ARGS} && make -k V=0 base-toolchain && make -k V=0 SAGE_SPKG="sage-spkg -y -o" SAGE_CHECK=warn SAGE_CHECK_PACKAGES="!cython,!r,!python3,!nose,!pathpy,!gap,!cysignals,!linbox,!git,!ppl,!cmake,!networkx,!symengine_py" {env:TARGETS_PRE:} {posargs:build} && (make -k V=0 SAGE_SPKG="sage-spkg -y -o" SAGE_CHECK=warn SAGE_CHECK_PACKAGES="!cython,!r,!python3,!nose,!pathpy,!gap,!cysignals,!linbox,!git,!ppl,!cmake" {env:TARGETS_OPTIONAL:} || echo "(error ignored)" ) '
 
 [testenv:check_configure]
 ## Test that configure behaves properly