ci: downgrade libm for MSRV testing

[cuviper]

bors r+

[bors[bot]]

Build failed:

• Test (1.44.0)

[cuviper]

bors retry

[bors[bot]]

Build failed:

• Test (1.44.0)

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• Format
• No Std
• Test (1.31.0)
• Test (1.35.0)
• Test (1.37.0)
• Test (1.38.0)
• Test (1.44.0)
• Test (1.46.0)
• Test (beta)
• Test (i586)
• Test (nightly)
• Test (stable)